# 100 leading professional services firms in Illinois

Tamtam tracks 339 professional services firms headquartered in Illinois. This list ranks the 100 largest by number of employees on LinkedIn, led by JLL, Cushman & Wakefield and Gallagher. Their median headcount is 1,505. Half of them were founded in 1987 or later. The most common headquarters are Chicago (72), Northbrook (4) and Naperville (3).

These are large professional services firms based in Illinois, primarily in the Chicago metropolitan area. The companies offer a wide range of specialized services, including real estate, insurance, accounting, IT consulting, and legal advice. They serve diverse clients from global corporations to middle-market businesses.

Key companies include JLL, a real estate and investment management firm, and Cushman & Wakefield, another global commercial real estate services provider. Gallagher and HUB International specialize in insurance and risk management. RSM US LLP, BDO USA, and Baker Tilly US are prominent accounting firms. CDW and Thoughtworks provide IT solutions and consulting, while Baker McKenzie and Kirkland & Ellis are major law practices. A notable pattern is the concentration of headquarters in Chicago, indicating a strong hub for professional services.

🇺🇸 Illinois · 100 companies · Updated Nov 11, 2026

| # | Company | Industry | Headquarters | Employees | Founded |
|---|---|---|---|---|---|
| 1 | [JLL](https://www.tamtam.ai/c/jll) (jill.com)<br>A leading professional services firm specializing in real estate and investment management, creating rewarding opportunities and sustainable solutions using advanced technology | Real Estate | 🇺🇸 Chicago | 104,899 |  |
| 2 | [Cushman & Wakefield](https://www.tamtam.ai/c/cushman-and-wakefield) (cushmanwakefield.com)<br>Global commercial real estate services provider with 52,000 professionals focused on advancing client, colleague, and community outcomes worldwide. | Real Estate | 🇺🇸 Chicago | 53,000 | 1917 |
| 3 | [Gallagher](https://www.tamtam.ai/c/gallagher) (gallagher.com)<br>Global insurance brokerage offering risk management, consulting, employee benefits, claims management, and retirement services to protect assets and support clients. | Insurance | 🇺🇸 Rolling Meadows | 52,767 |  |
| 4 | [RSM US LLP](https://www.tamtam.ai/c/rsm-us-llp) (rsmus.com)<br>Provider of assurance, tax, and consulting services focused on middle market clients with a global presence and professional development culture. | Accounting | 🇺🇸 Chicago | 19,700 | 1926 |
| 5 | [CDW](https://www.tamtam.ai/c/cdw) (cdw.com)<br>A leading IT solutions provider offering a full-stack approach with extensive services and deep industry expertise to drive innovation and achieve organizational goals | IT Services and IT Consulting | 🇺🇸 Vernon Hills | 18,732 |  |
| 6 | [HUB International](https://www.tamtam.ai/c/hub-international) (hubinternational.com)<br>Advises businesses and individuals on insurance, risk management, employee benefits, retirement, and wealth management with tailored solutions. | Insurance | 🇺🇸 Chicago | 16,129 |  |
| 7 | [UL Solutions](https://www.tamtam.ai/c/ul-solutions) (ul.com)<br>Global provider of safety testing, certification, inspection, auditing, and advisory services ensuring product security, sustainability, and market trust. | Professional Services | 🇺🇸 Northbrook | 14,487 | 1894 |
| 8 | [Thoughtworks](https://www.tamtam.ai/c/thoughtworks) (thoughtworks.com)<br>A global technology consultancy specializing in strategy, design, and engineering to drive digital innovation, fostering diverse talent and a strong cultivation culture | IT Services and IT Consulting | 🇺🇸 Chicago | 12,735 |  |
| 9 | [Baker McKenzie](https://www.tamtam.ai/c/baker-mckenzie) (bakermckenzie.com)<br>Provide practical legal advice combining local knowledge and international expertise to help businesses succeed across global markets. | Law Practice | 🇺🇸 Chicago | 11,895 |  |
| 10 | [Grant Thornton (US)](https://www.tamtam.ai/c/grant-thornton-us) (grantthornton.com)<br>Provides independent audit, tax, and advisory services through a global network, focusing on quality, trust, and strong client relationships. | Professional Services | 🇺🇸 Chicago | 11,518 | 1924 |
| 11 | [BDO USA](https://www.tamtam.ai/c/bdo-usa) (bdo.com)<br>Providing middle market leaders with assurance, tax, and advisory services focused on cultivating a people-centered, growth-oriented corporate culture. | Accounting | 🇺🇸 Chicago | 11,182 | 1910 |
| 12 | [Baker Tilly US](https://www.tamtam.ai/c/baker-tilly-us) (bakertilly.com)<br>A leading advisory CPA firm with a coast-to-coast and global presence, part of Baker Tilly International network with 43,000 professionals | Accounting | 🇺🇸 Chicago | 10,594 |  |
| 13 | [Crowe](https://www.tamtam.ai/c/crowe-chicago) (crowe.com)<br>A global firm providing public accounting, consulting, and technology services, helping clients make smart decisions and uncover hidden opportunities worldwide | Professional Services | 🇺🇸 Chicago | 9,411 |  |
| 14 | [Kirkland & Ellis](https://www.tamtam.ai/c/kirkland-and-ellis) (kirkland.com)<br>Providing top-tier legal services in corporate transactions, litigation, investigations, and asset management with global reach and innovative approach. | Law Practice | 🇺🇸 Chicago | 8,683 | 1909 |
| 15 | [Kearney](https://www.tamtam.ai/c/kearney) (kearney.com)<br>Global consulting firm with a focus on people, strategic operations, and building the future through teamwork and hands-on expertise in over 40 countries | Business Consulting and Services | 🇺🇸 Chicago | 7,559 | 1926 |
| 16 | [Gallagher Bassett](https://www.tamtam.ai/c/gallagher-bassett) (gallagherbassett.com)<br>Provider of claims and risk management services, offering consulting, technical support, and innovative solutions across global insurance sectors. | Insurance | 🇺🇸 Rolling Meadows | 6,826 | 1962 |
| 17 | [Leo Burnett](https://www.tamtam.ai/c/leo-burnett) (leoburnett.com)<br>Global communications agency combining creativity, data, and technology to influence human behavior and drive client prosperity worldwide. | Advertising Services | 🇺🇸 Chicago | 6,825 |  |
| 18 | [Huron](https://www.tamtam.ai/c/huron) (huronconsultinggroup.com)<br>A global professional services firm specializing in strategy creation, operations optimization, digital transformation, and empowering businesses for sustainable results | Business Consulting and Services | 🇺🇸 Chicago | 6,453 | 2002 |
| 19 | [Accenture Strategy & Consulting](https://www.tamtam.ai/c/accenture-strategy-and-consulting) (accenture.com)<br>Combines deep industry expertise with advanced analytics and human-led design methodologies to help global clients act with speed and confidence throughout operations. | Business Consulting and Services | 🇺🇸 Chicago | 6,217 |  |
| 20 | [Sidley Austin LLP](https://www.tamtam.ai/c/sidley-austin-llp) (sidley.com)<br>Providing comprehensive legal services worldwide, specializing in complex transactions, restructuring, investigations, regulatory, and litigation matters across multiple industries. | Law Practice | 🇺🇸 Chicago | 5,113 | 1866 |
| 21 | [Mayer Brown](https://www.tamtam.ai/c/mayer-brown) (mayerbrown.com)<br>Representation of major corporations, funds, and financial institutions in complex transactions and disputes across international markets. | Law Practice | 🇺🇸 Chicago | 4,544 |  |
| 22 | [Sargent & Lundy](https://www.tamtam.ai/c/sargent-and-lundy) (sargentlundy.com)<br>A global leader in architect engineering specializing in power, energy, and decarbonization, offering comprehensive project services to public and private clients | Professional Services | 🇺🇸 Chicago | 4,511 | 1891 |
| 23 | [AHEAD](https://www.tamtam.ai/c/ahead-chicago) (ahead.com)<br>This company specializes in building and managing digital platforms for successful organizations, accelerating the impact of technology through innovative solutions and engineering expertise | IT Services and IT Consulting | 🇺🇸 Chicago | 4,223 |  |
| 24 | [Spencer Stuart](https://www.tamtam.ai/c/spencer-stuart) (spencerstuart.com)<br>Specializes in senior executive search, board recruitment, leadership advisory, succession planning, and organizational effectiveness for diverse global clients. | Business Consulting and Services | 🇺🇸 Chicago | 3,609 |  |
| 25 | [Chartis](https://www.tamtam.ai/c/chartis) (chartis.com)<br>Consulting firm specializing in healthcare strategy, transformation, and operational models to improve affordability, accessibility, safety, and patient experience. | Business Consulting and Services | 🇺🇸 Chicago | 3,598 | 2001 |
| 26 | [Mu Sigma Inc.](https://www.tamtam.ai/c/mu-sigma-inc) (mu-sigma.com)<br>A leading Big Data analytics company partnering with Fortune 500 firms to enhance decision-making, recognized for innovation and transformative solutions | Business Consulting and Services | 🇺🇸 Northbrook | 3,563 | 2004 |
| 27 | [Cushman & Wakefield - Formerly DTZ](https://www.tamtam.ai/c/cushman-and-wakefield-formerly-dtz) (cushmanwakefield.com)<br>A global leader in commercial real estate services, providing insights and value for occupiers and investors worldwide through various core services | Commercial Real Estate | 🇺🇸 Chicago | 3,518 |  |
| 28 | [Egen](https://www.tamtam.ai/c/egen) (egen.ai)<br>A merged company combining platform engineering with data, AI, analytics, and cloud expertise to empower organizations in their digital journeys | IT Services and IT Consulting | 🇺🇸 Naperville | 3,096 | 2000 |
| 29 | [Perkins&Will](https://www.tamtam.ai/c/perkins-and-will) (perkinswill.com)<br>Design and architecture firm creating healthy, beautiful spaces worldwide with expertise in architecture, interior design, urban and landscape planning. | Architecture and Planning | 🇺🇸 Chicago | 3,060 | 1935 |
| 30 | [Staff Management \| SMX](https://www.tamtam.ai/c/staff-management-smx) (staffmanagement.com)<br>A leader in workforce solutions for fulfillment, distribution, and manufacturing industries, specializing in Onsite Workforce Management and Recruitment and Placement Solutions | Staffing and Recruiting | 🇺🇸 Chicago | 3,025 | 1988 |
| 31 | [Intellias](https://www.tamtam.ai/c/intellias) (intellias.com)<br>Trusted technology partner accelerating sustainable digitalization for top-tier organizations, delivering mission-critical projects and measurable outcomes for clients globally | IT Services and IT Consulting | 🇺🇸 Chicago | 2,955 | 2002 |
| 32 | [ENTRUST Solutions Group](https://www.tamtam.ai/c/entrust-solutions-group) (entrustsol.com)<br>Providing engineering, consulting, and automation services to pipeline, utility, and industrial sectors with expertise in various areas across multiple office locations | Utilities | 🇺🇸 Lisle | 2,943 | 2002 |
| 33 | [Credico](https://www.tamtam.ai/c/credico) (credico.com)<br>A leading customer acquisition agency connecting brands with local sales teams to acquire new customers through face-to-face methods | Advertising Services | 🇺🇸 Chicago | 2,779 | 1991 |
| 34 | [Clayco](https://www.tamtam.ai/c/clayco) (claycorp.com)<br>A full-service real estate development, architecture, engineering, and construction firm delivering high-quality solutions within North America on time and on budget | Construction | 🇺🇸 Chicago | 2,744 | 1984 |
| 35 | [Heidrick & Struggles](https://www.tamtam.ai/c/heidrick-and-struggles) (heidrick.com)<br>Providing executive search, leadership consulting, and corporate culture services to global Fortune 1000 companies for improved performance. | Business Consulting and Services | 🇺🇸 Chicago | 2,477 | 1953 |
| 36 | [West Monroe](https://www.tamtam.ai/c/west-monroe) (westmonroe.com)<br>Global consulting firm specializing in business and technology solutions, focusing on rapid impact through industry, strategy, and people integration. | Business Consulting and Services | 🇺🇸 Chicago | 2,416 | 2002 |
| 37 | [Sikich](https://www.tamtam.ai/c/sikich) (sikich.com)<br>A global professional services company specializing in technology solutions, delivering transformative digital strategies and financial services to various clients worldwide | Business Consulting and Services | 🇺🇸 Chicago | 2,335 | 1982 |
| 38 | [Seyfarth Shaw LLP](https://www.tamtam.ai/c/seyfarth-shaw-llp) (seyfarth.com)<br>Specialized legal firm providing innovative, high-caliber representation in complex litigation, transformational deals, and cross-border projects globally. | Law Practice | 🇺🇸 Chicago | 2,326 | 1945 |
| 39 | [Medix™](https://www.tamtam.ai/c/medix-chicago) (medixteam.com)<br>Provides workforce solutions and creates opportunities for talent in healthcare, life sciences, engineering, and technology sectors. | Staffing and Recruiting | 🇺🇸 Chicago | 2,319 | 2001 |
| 40 | [NORC at the University of Chicago](https://www.tamtam.ai/c/norc-at-the-university-of-chicago) (norc.org)<br>An objective research organization partnering with clients to inform decisions on health, education, economy, and government programs for a better world | Research Services | 🇺🇸 Chicago | 2,285 | 1941 |
| 41 | [IMEG](https://www.tamtam.ai/c/imeg) (imegcorp.com)<br>IMEG is a national engineering and design consulting company focusing on people-centered engineering, high-performing building systems, infrastructure, and construction services | Design Services | 🇺🇸 Rock Island | 2,268 | 1955 |
| 42 | [Zifo](https://www.tamtam.ai/c/zifo) (zifornd.com)<br>Provides specialist scientific informatics services including digital transformation, data science, AI, and project management for science-driven organizations. | IT Services and IT Consulting | 🇺🇸 Deerfield | 2,214 | 2008 |
| 43 | [Addison Group](https://www.tamtam.ai/c/addison-group) (addisongroup.com)<br>Professional services firm specializing in talent solutions, consulting, recruiting, and executive search across various sectors, recognized for industry excellence and growth | Staffing and Recruiting | 🇺🇸 Chicago | 2,130 | 1999 |
| 44 | [Quinnox](https://www.tamtam.ai/c/quinnox) (quinnox.com)<br>A platform and solutions company utilizing human and applied intelligence to simplify business processes, improve customer experiences, and create exceptional business value | IT Services and IT Consulting | 🇺🇸 Chicago | 2,023 | 2002 |
| 45 | [MathCo](https://www.tamtam.ai/c/mathco) (mathco.com)<br>Global leader in Enterprise AI and Analytics, trusted by top organizations Specializing in custom data products, advanced analytics, and connected intelligence solutions | Business Consulting and Services | 🇺🇸 Chicago | 1,954 | 2016 |
| 46 | [Winston & Strawn LLP](https://www.tamtam.ai/c/winston-and-strawn-llp) (winstontaylor.com)<br>Provides comprehensive legal services including litigation, transactions, regulatory advice, and business solutions with a focus on client partnership and inclusion. | Law Practice | 🇺🇸 Chicago | 1,752 |  |
| 47 | [Cresa](https://www.tamtam.ai/c/cresa-chicago) (cresa.com)<br>A global advisory firm specializing in representing occupiers, offering integrated real estate solutions, transaction management, workplace solutions, project management, consulting, and more | Real Estate | 🇺🇸 Chicago | 1,694 | 1993 |
| 48 | [Bitwise](https://www.tamtam.ai/c/bitwise) (bitwiseglobal.com)<br>Provides automated ETL, data migration, AI/ML, and BI solutions leveraging 28+ years of experience to accelerate analytics and AI readiness. | IT Services and IT Consulting | 🇺🇸 Schaumburg | 1,582 | 1996 |
| 49 | [Katten Muchin Rosenman LLP](https://www.tamtam.ai/c/katten-muchin-rosenman-llp) (katten.com)<br>A full-service law firm with nearly 700 attorneys offering high-value legal services globally, specializing in corporate, financial services, litigation, real estate, and more | Law Practice | 🇺🇸 Chicago | 1,550 | 1974 |
| 50 | [Navigant](https://www.tamtam.ai/c/navigant) (guidehouse.com)<br>Global provider of management technology and risk consulting services helping public and commercial clients address transformational change technology innovation and regulatory pressure | Business Consulting and Services | 🇺🇸 Chicago | 1,505 |  |
| 51 | [Advanced Group](https://www.tamtam.ai/c/advanced-group) (advancedgroup.com)<br>Specializing in talent acquisition, outsourcing, and professional services, we provide strategic resourcing, staffing, consulting, and recruitment process outsourcing to various industries | Staffing and Recruiting | 🇺🇸 Chicago | 1,414 |  |
| 52 | [CCMSI](https://www.tamtam.ai/c/ccmsi) (ccmsi.com)<br>A leading third-party administrator specializing in workers' compensation and property/casualty self-insurance programs with a focus on efficient, cost-effective claims management nationwide | Insurance | 🇺🇸 Danville | 1,391 | 1978 |
| 53 | [Royal Cyber Inc.](https://www.tamtam.ai/c/royal-cyber-inc) (royalcyber.com)<br>Empowering enterprises to evolve with agility, security, and innovation through digital transformation and modernization of legacy systems with AI-driven solutions. | IT Services and IT Consulting | 🇺🇸 Naperville | 1,383 | 2002 |
| 54 | [Mérieux NutriSciences - North America](https://www.tamtam.ai/c/merieux-nutrisciences-north-america) (mxns.com)<br>Global food safety partner providing testing, labeling, auditing, consulting, and research services to the food industry worldwide, focusing on consumer health and scientific excellence | Biotechnology Research | 🇺🇸 Chicago | 1,301 | 1967 |
| 55 | [Benesch](https://www.tamtam.ai/c/benesch) (benesch.com)<br>A leading company shaping infrastructure and communities, focused on client needs and delivering project success with top talent | Professional Services | 🇺🇸 Chicago | 1,286 | 1946 |
| 56 | [Hinshaw & Culbertson LLP](https://www.tamtam.ai/c/hinshaw-and-culbertson-llp) (hinshawlaw.com)<br>A US law firm with offices nationwide and in London specializing in insurance, professional services, banking, finance, debt collection, and litigation services | Law Practice | 🇺🇸 Chicago | 1,274 | 1934 |
| 57 | [The Mom Project](https://www.tamtam.ai/c/the-mom-project) (themomproject.com)<br>Empowering moms to succeed in the workforce by recognizing their diverse talents and skills, ultimately benefiting families, businesses, and society | Staffing and Recruiting | 🇺🇸 Chicago | 1,242 | 2015 |
| 58 | [Jenner & Block](https://www.tamtam.ai/c/jenner-and-block) (jenner.com)<br>Jenner & Block is a renowned law firm with over 500 lawyers known for fearless advocacy, creative solutions, and shaping legal outcomes | Law Practice | 🇺🇸 Chicago | 1,226 | 1914 |
| 59 | [DGS Technical Services Inc.](https://www.tamtam.ai/c/dgs-technical-services-inc) (dgsts.com)<br>Structural steel design, detailing, and mechanical engineering services provider with expertise in various disciplines and offshore delivery capabilities in multiple countries | Design Services | 🇺🇸 Elgin | 1,217 | 2004 |
| 60 | [PeopleScout](https://www.tamtam.ai/c/peoplescout) (peoplescout.com)<br>Global talent solutions provider offering RPO, MSP, and total workforce solutions with AI-driven technology, data insights, and a focus on efficiency and cost reduction | Staffing and Recruiting | 🇺🇸 Chicago | 1,172 | 1992 |
| 61 | [ARi](https://www.tamtam.ai/c/ari) (arigs.com)<br>Global engineering firm offering services in Embedded Software, Product Engineering, Manufacturing Engineering, Testing & Validation, Automation & Robotics, and Digital Transformation to industry leaders worldwide | Industrial Machinery Manufacturing | 🇺🇸 East Peoria | 1,170 | 2006 |
| 62 | [66degrees](https://www.tamtam.ai/c/66degrees) (66degrees.com)<br>A leading AI and Data consulting company specializing in developing AI-focused solutions using the latest advancements in AI, data, and cloud technology | Business Consulting and Services | 🇺🇸 Chicago | 1,133 | 2012 |
| 63 | [Advanced Clinical](https://www.tamtam.ai/c/advanced-clinical) (advancedclinical.com)<br>clinical development and strategic resourcing organization improving clinical research experiences and outcomes through customized solutions and experienced guidance | Biotechnology Research | 🇺🇸 Deerfield | 1,118 |  |
| 64 | [Clune Construction Company](https://www.tamtam.ai/c/clune-construction-company) (clunegc.com)<br>A national, employee-owned general contractor providing construction management services, focused on ethical practices, employee satisfaction, and innovation for clients | Construction | 🇺🇸 Chicago | 1,107 | 1997 |
| 65 | [HORNE](https://www.tamtam.ai/c/horne) (horne.com)<br>Professional services firm supporting clients in public, private & government sectors nationwide, empowering team members to serve with empathy and clarity | Professional Services | 🇺🇸 Chicago | 1,083 | 1962 |
| 66 | [Harbor](https://www.tamtam.ai/c/harbor-chicago) (harborglobal.com)<br>A global provider of expert services in strategy, legal technology, operations, and intelligence for leading law firms, corporations, and law departments | Business Consulting and Services | 🇺🇸 Chicago | 1,072 |  |
| 67 | [K2 Services](https://www.tamtam.ai/c/k2-services) (opensitysolutions.com)<br>Provider of IT consulting and support services for the legal community, specializing in document management, application support, networking, security, training, and help desk support | IT Services and IT Consulting | 🇺🇸 Chicago | 998 | 1985 |
| 68 | [Lochner](https://www.tamtam.ai/c/lochner) (hwlochner.com)<br>Trusted infrastructure services provider for aviation, surface transportation, and water clients nationwide, driven by values, collaboration, and excellence | Civil Engineering | 🇺🇸 Chicago | 960 | 1944 |
| 69 | [CIeNET Technologies](https://www.tamtam.ai/c/cienet-technologies) (cienet.com)<br>A leading technology service provider offering technical consulting, software engineering, and solution services to Fortune 500 companies globally with top-notch quality assurance processes | IT Services and IT Consulting | 🇺🇸 Oak Brook | 928 | 2000 |
| 70 | [Impact Advisors](https://www.tamtam.ai/c/impact-advisors) (impact-advisors.com)<br>Healthcare management consulting firm solving industry challenges with team of experts, quality solutions, and measurable value Award-winning and innovative culture | Hospitals and Health Care | 🇺🇸 Naperville | 927 | 2007 |
| 71 | [JSG (Johnson Service Group, Inc.)](https://www.tamtam.ai/c/jsg-johnson-service-group-inc) (jsginc.com)<br>A leader in staffing and consulting services with 40 years of experience, offering world-class service, diverse offerings, and IT consulting | Staffing and Recruiting | 🇺🇸 Westmont | 923 | 1984 |
| 72 | [Wiss, Janney, Elstner Associates, Inc.](https://www.tamtam.ai/c/wiss-janney-elstner-associates-inc) (wje.com)<br>Global firm of engineers, architects, and materials scientists providing innovative solutions for problem-solving, repairs, and prevention in the built world | Civil Engineering | 🇺🇸 Northbrook | 909 | 1956 |
| 73 | [DHR Global](https://www.tamtam.ai/c/dhr-global) (dhrglobal.com)<br>Global leadership consulting and executive search provider across twenty industries, offering talent advisory and emerging leader solutions through sixty worldwide market locations. | Business Consulting and Services | 🇺🇸 Chicago | 907 | 1989 |
| 74 | [Vedder](https://www.tamtam.ai/c/vedder) (vedderprice.com)<br>A global law firm with over 400 professionals providing high-value legal counsel in M&A, banking, litigation, executive compensation, employment law, and more | Law Practice | 🇺🇸 Chicago | 899 | 1952 |
| 75 | [ARMStrong Insurance Services](https://www.tamtam.ai/c/armstrong-insurance-services) (armsrm.com)<br>A leading parent company in debt recovery and financial solutions serving businesses with efficient B2B collections and tailored debt recovery services | Financial Services | 🇺🇸 Itasca | 859 |  |
| 76 | [HBK Engineering, LLC](https://www.tamtam.ai/c/hbk-engineering-llc) (hbkengineering.com)<br>A professional engineering design team providing civil, environmental, structural, electrical, and utility services with a focus on accurate, time-sensitive projects | Civil Engineering | 🇺🇸 Chicago | 838 | 1999 |
| 77 | [Kay and Associates, Inc.](https://www.tamtam.ai/c/kay-and-associates-inc) (kayinc.com)<br>Established in 1960, providing high quality engineering and technical services to industry and government worldwide, specializing in maintenance and contract management | Airlines and Aviation | 🇺🇸 Buffalo Grove | 821 | 1960 |
| 78 | [Locke Lord LLP](https://www.tamtam.ai/c/locke-lord-llp) (troutman.com)<br>Premier full-service law firm specializing in complex litigation, regulatory, and transactional work for clients globally, with a focus on diverse industries | Law Practice | 🇺🇸 Chicago | 816 | 1887 |
| 79 | [Professional Service Industries, Inc. (PSI)](https://www.tamtam.ai/c/professional-service-industries-inc-psi) (intertek.com)<br>A leading provider of quality solutions in the construction industry, offering integrated engineering, testing, and assurance services to a broad customer base | Civil Engineering | 🇺🇸 Arlington Heights | 810 | 1961 |
| 80 | [Baker Tilly One India](https://www.tamtam.ai/c/baker-tilly-one-india) (mossadams.com)<br>Global expansion of a leading accounting and consulting firm with a focus on delivering high-quality services, innovation, and developing future leaders | Professional Services | 🇺🇸 Chicago | 807 | 1931 |
| 81 | [TopBloc](https://www.tamtam.ai/c/topbloc) (topbloc.com)<br>A technology consulting firm specializing in fixed-price Workday deployment and support services, using automation technology to streamline processes for clients | IT Services and IT Consulting | 🇺🇸 Chicago | 797 | 2016 |
| 82 | [Palni Inc](https://www.tamtam.ai/c/palni-inc) (palni.com)<br>A technocratic company specializing in Salesforce consultation, implementation, and integration to boost clients' revenue by solving complex problems and providing IT strategy | IT Services and IT Consulting | 🇺🇸 Schaumburg | 770 | 2008 |
| 83 | [UIT](https://www.tamtam.ai/c/uit) (uitonline.com)<br>This company based in Chicago offers comprehensive solutions for telecommunications companies, including consulting, staffing services, training, and education programs nationwide | IT Services and IT Consulting | 🇺🇸 Algonquin | 729 | 1987 |
| 84 | [LaSalle Network](https://www.tamtam.ai/c/lasalle-network) (thelasallenetwork.com)<br>Specializes in staffing and recruiting across multiple industries, placing candidates in temporary and permanent roles nationwide since 1998. | Staffing and Recruiting | 🇺🇸 Chicago | 724 | 1998 |
| 85 | [The Second City](https://www.tamtam.ai/c/the-second-city) (secondcity.com)<br>This professional services company uses comedy theater methods to create engaging events, content, and development programs that inspire better performance | Professional Training and Coaching | 🇺🇸 Chicago | 712 | 1959 |
| 86 | [Buck](https://www.tamtam.ai/c/buck-rolling-meadows) (ajg.com)<br>An integrated firm providing HR, pensions, and employee benefits consulting, technology, and administration services to organizations, trustees, and employees for secure futures and improved performance | Financial Services | 🇺🇸 Rolling Meadows | 710 |  |
| 87 | [Smithbucklin](https://www.tamtam.ai/c/smithbucklin) (smithbucklin.com)<br>Providing comprehensive association management, event coordination, educational programming, and strategic consulting to modernize operations, advance organizational missions, and deliver enhanced member value. | Non-profit Organizations | 🇺🇸 Chicago | 708 | 1949 |
| 88 | [LCW](https://www.tamtam.ai/c/lcw) (lcwinclusion.com)<br>LCW helps organizations worldwide thrive in culturally diverse environments through DEI and culture transformation consulting, training, and translation services | Professional Training and Coaching | 🇺🇸 Chicago | 704 | 2000 |
| 89 | [O'Hagan Meyer](https://www.tamtam.ai/c/o-hagan-meyer) (ohaganmeyer.com)<br>A national law firm specializing in defending corporations in litigation cases, providing tailored services for clients across multiple locations nationwide | Law Practice | 🇺🇸 Chicago | 686 |  |
| 90 | [Trustwave, A LevelBlue Company](https://www.tamtam.ai/c/trustwave-a-levelblue-company) (levelblue.com)<br>Reduces cyber risk, fortifies organizations against threats, detects what others cannot, responds with speed, optimizes cyber investment, improves security resilience. | Computer and Network Security | 🇺🇸 Chicago | 673 | 1995 |
| 91 | [Revantage, A Blackstone Portfolio Company](https://www.tamtam.ai/c/revantage-a-blackstone-portfolio-company) (revantage.com)<br>A global leader in corporate services catering to Blackstone Real Estate portfolio companies and investments worldwide | Real Estate | 🇺🇸 Chicago | 666 | 2017 |
| 92 | [SCB](https://www.tamtam.ai/c/scb) (scb.com)<br>A leading architecture firm offering design services for urban living, workplaces, learning environments, infrastructure, and communities with a century-long legacy | Architecture and Planning | 🇺🇸 Chicago | 659 | 1931 |
| 93 | [Spaulding Ridge](https://www.tamtam.ai/c/spaulding-ridge) (spauldingridge.com)<br>Global leader in business transformation for CFO and CRO offices Specializing in improving insight, collaboration, and decision-making through cloud technology solutions for Fortune 500 companies | Business Consulting and Services | 🇺🇸 Chicago | 658 | 2018 |
| 94 | [Farnsworth Group, Inc.](https://www.tamtam.ai/c/farnsworth-group-inc) (f-w.com)<br>Employee-owned design firm with over 500 professionals offering architecture, engineering, surveying, and consulting services to create lasting relationships and deliver exceptional design solutions nationwide | Design Services | 🇺🇸 Bloomington | 645 |  |
| 95 | [Kaufman Hall](https://www.tamtam.ai/c/kaufman-hall) (kaufmanhall.com)<br>The company specializes in providing management consulting services to healthcare and higher education institutions to optimize their financial planning and performance | Business Consulting and Services | 🇺🇸 Chicago | 643 |  |
| 96 | [Vestian](https://www.tamtam.ai/c/vestian) (vestian.com)<br>Vestian provides conflict-free real estate services, prioritizing occupier interests, offering strategic guidance, brokerage, design & build, and management solutions globally | Real Estate | 🇺🇸 Chicago | 627 | 2010 |
| 97 | [Hilco Global](https://www.tamtam.ai/c/hilco-global) (hilcoglobal.com)<br>Global independent financial services firm providing valuation, monetization, advisory, and capital solutions for tangible and intangible assets, specializing in maximizing value for companies | Financial Services | 🇺🇸 Northbrook | 626 |  |
| 98 | [Enventure Engineering](https://www.tamtam.ai/c/enventure-engineering) (enventure.com)<br>A leading Engineering, Supply Chain, and Compliance solutions provider with expertise in engineering services, master data management, asset management, and compliance solutions | Engineering Services | 🇺🇸 Chicago | 603 | 1997 |
| 99 | [Cook County Public Defender](https://www.tamtam.ai/c/cook-county-public-defender) (cookcountypublicdefender.org)<br>A leading criminal defense firm in Cook County, Illinois, providing legal representation to thousands of individuals facing criminal charges and child protection violations | Legal Services | 🇺🇸 Chicago | 596 |  |
| 100 | [Hanson Professional Services Inc.](https://www.tamtam.ai/c/hanson-professional-services-inc) (hanson-inc.com)<br>Employee-owned consulting firm offering engineering, planning, and allied services, with a focus on client success through hands-on project management | Civil Engineering | 🇺🇸 Springfield | 594 | 1954 |

## How this list is built

Every company on this list has a Tamtam company profile. The industry is the one the company gives on LinkedIn; categories such as SaaS or fintech are classified by Tamtam's AI from the company's website and LinkedIn page; and the headquarters city must match the company's own address. Companies are ranked by the number of employees on LinkedIn, and the list is refreshed every night. Classifications can be wrong, so treat the list as a starting point rather than an official ranking. Last updated Nov 11, 2026.

## More company lists in Illinois

- [The largest companies in Illinois](https://www.tamtam.ai/l/largest-companies-illinois) (4,687)
- [Multinational companies in Illinois](https://www.tamtam.ai/l/multinational-companies-illinois) (831)
- [Manufacturers in Illinois](https://www.tamtam.ai/l/manufacturers-illinois) (528)
- [Education providers in Illinois](https://www.tamtam.ai/l/education-companies-illinois) (370)
- [Financial services firms in Illinois](https://www.tamtam.ai/l/financial-services-firms-illinois) (352)
- [Companies with in-house R&D in Illinois](https://www.tamtam.ai/l/r-and-d-companies-illinois) (322)
- [Hardware manufacturers in Illinois](https://www.tamtam.ai/l/hardware-companies-illinois) (296)
- [IT Services and IT Consulting companies in Illinois](https://www.tamtam.ai/l/it-services-and-it-consulting-companies-illinois) (294)
- [Hospitals and Health Care companies in Illinois](https://www.tamtam.ai/l/hospitals-and-health-care-companies-illinois) (280)
- [IT services companies in Illinois](https://www.tamtam.ai/l/it-services-companies-illinois) (274)
- [Healthcare providers in Illinois](https://www.tamtam.ai/l/healthcare-providers-illinois) (262)
- [Retail chains in Illinois](https://www.tamtam.ai/l/retail-chains-illinois) (260)
- [Financial Services companies in Illinois](https://www.tamtam.ai/l/financial-services-companies-illinois) (248)
- [Critical infrastructure operators in Illinois](https://www.tamtam.ai/l/critical-infrastructure-operators-illinois) (241)
- [Consulting firms in Illinois](https://www.tamtam.ai/l/consulting-firms-illinois) (238)
- [Logistics companies in Illinois](https://www.tamtam.ai/l/logistics-companies-illinois) (228)
- [Transportation, Logistics, Supply Chain and Storage companies in Illinois](https://www.tamtam.ai/l/transportation-logistics-supply-chain-and-storage-companies-illinois) (215)
- [SaaS companies in Illinois](https://www.tamtam.ai/l/saas-companies-illinois) (182)
- [Companies with a mobile app in Illinois](https://www.tamtam.ai/l/companies-with-mobile-app-illinois) (178)
- [Family-owned companies in Illinois](https://www.tamtam.ai/l/family-owned-companies-illinois) (166)
- [Subscription businesses in Illinois](https://www.tamtam.ai/l/subscription-businesses-illinois) (160)
- [Corporate groups and holding companies in Illinois](https://www.tamtam.ai/l/holding-companies-illinois) (154)
- [Publicly traded companies in Illinois](https://www.tamtam.ai/l/publicly-traded-companies-illinois) (154)
- [Public sector organisations in Illinois](https://www.tamtam.ai/l/public-sector-organisations-illinois) (147)

## The same list elsewhere in United States

- [Professional services firms in the United States](https://www.tamtam.ai/l/professional-services-firms-united-states) (7,078)
- [Professional services firms in New York State](https://www.tamtam.ai/l/professional-services-firms-new-york-state) (955)
- [Professional services firms in California](https://www.tamtam.ai/l/professional-services-firms-california) (880)
- [Professional services firms in Texas](https://www.tamtam.ai/l/professional-services-firms-texas) (629)
- [Professional services firms in Florida](https://www.tamtam.ai/l/professional-services-firms-florida) (375)
- [Professional services firms in Virginia](https://www.tamtam.ai/l/professional-services-firms-virginia) (344)
- [Professional services firms in Massachusetts](https://www.tamtam.ai/l/professional-services-firms-massachusetts) (249)
- [Professional services firms in New Jersey](https://www.tamtam.ai/l/professional-services-firms-new-jersey) (249)
- [Professional services firms in Georgia](https://www.tamtam.ai/l/professional-services-firms-georgia) (246)
- [Professional services firms in Pennsylvania](https://www.tamtam.ai/l/professional-services-firms-pennsylvania) (245)
- [Professional services firms in Ohio](https://www.tamtam.ai/l/professional-services-firms-ohio) (195)
- [Professional services firms in Maryland](https://www.tamtam.ai/l/professional-services-firms-maryland) (176)
- [Professional services firms in Michigan](https://www.tamtam.ai/l/professional-services-firms-michigan) (172)
- [Professional services firms in Washington State](https://www.tamtam.ai/l/professional-services-firms-washington-state) (163)
- [Professional services firms in North Carolina](https://www.tamtam.ai/l/professional-services-firms-north-carolina) (142)
- [Professional services firms in Alabama](https://www.tamtam.ai/l/professional-services-firms-alabama) (135)
- [Professional services firms in Minnesota](https://www.tamtam.ai/l/professional-services-firms-minnesota) (125)
- [Professional services firms in New York](https://www.tamtam.ai/l/professional-services-firms-new-york) (735)
- [Professional services firms in Chicago](https://www.tamtam.ai/l/professional-services-firms-chicago) (196)
- [Professional services firms in Houston](https://www.tamtam.ai/l/professional-services-firms-houston) (156)
- [Professional services firms in Atlanta](https://www.tamtam.ai/l/professional-services-firms-atlanta) (143)
- [Professional services firms in San Francisco](https://www.tamtam.ai/l/professional-services-firms-san-francisco) (130)
- [Professional services firms in Washington](https://www.tamtam.ai/l/professional-services-firms-washington) (109)
- [Professional services firms in Los Angeles](https://www.tamtam.ai/l/professional-services-firms-los-angeles) (108)

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Source: https://www.tamtam.ai/l/professional-services-firms-illinois