Below is a list of the 25 top consulting firms. While there's some room for debate as to who belongs in the final spots (varying based upon geography and specialization), there's a pretty good consensus on who belongs on top.
1) McKinsey & Company
Many regard McKinsey as the firm that started the whole strategy consulting industry. Headquartered in New York with 17,000 employees spread across almost 100 offices worldwide, they've got a level of prestige that makes them desired by graduates from top schools worldwide.
2) The Boston Consulting Group
BCG is headquartered in Boston, with over 70 offices and 4,500 employees. They do work similar to McKinsey and Bain and are known for having an approach that's slightly more academic, emphasizing thought leadership more than the other two.
3) Bain & Company
Bain is the final member of the MBB (McKinsey-BCG-Bain) prestige trio and it competes for the similar work from Corporate boards. Headquartered in Boston, with 5,000 employees, their number one priority is client financial results.
4) Booz & Company
Booz & Company (often abbreviated 'Booz and Co') used to have three names in the firm ("Booz Allen Hamilton") but is now independent of the US government practices that previously dominated Booz Allen Hamilton. Booz is headquartered in New York with 3,300 employees spread across 61 offices.
5) Monitor Group
The legendary creator of Porter's Five Forces, Michael Porter, co-founded this consultancy in 1983. The Cambridge, MA-based consultancy has now grown to 1,500 employees across 29 offices. (Note: Monitor was acquired by Deloitte in November, 2012.)
6) Deloitte Consulting
Deloitte Consulting is a part of the Big Four audit firm Deloitte Touche Tohmatsu. They consult in three key areas: human capital, strategy & operations, and technology. They have 15,000 employees in the consulting practice and are headquartered in New York with 50 offices worldwide.
7) Oliver Wyman
Oliver Wyman operates in over 50 cities with 3,000 employees. They draw on the expertise from other companies within their parent company, Marsh & McLennan. They're known for their financial capabilities and international acumen.
8.) Roland Berger
Unlike many top firms headquartered in the United States, Roland Berger touts its European heritage. Founded in Munich in 1967, they provide strategy consulting in 46 offices via over 2,000 employees.
L.E.K. tackles questions of strategy with 900 employees in 20 offices. They were founded in London in 1983. They emphasize five main capabilities: Strategy, Shareholder Value Management, M&A Support, Operations & Organization, and Marketing & Sales.
10) A.T. Kearney
Headquartered in Chicago, A.T. Kearney has a rich history tracing back to the days of McKinsey. The firm split from McKinsey in 1939 and has maintained a focus on operations, supply chain, and manufacturing.
11) ZS Associates
Founded by Northwestern professors, ZS Associates was founded in Evanston, IL and has grown to over 20 offices around the world. They mostly tackle projects related to sales and marketing. They serve a variety of clients, but pharmaceuticals are still their bread and butter.
Headquartered in New York, Accenture is the world's largest consulting firm, with revenues over $20 billion. It's primarily known for systems integration work, but they also have a small strategy consulting unit.
Headquartered in Paris, Capgemini serves clients in a wide variety of industries through outsourcing, IT, and management consulting projects. Their life sciences division is known to be particularly strong.
Headquartered in New York, PWC's consulting arm addresses issues related to finance, operations, forensic accounting, organizational structure, crisis management, and IT. Their engagements and focus largely emerge from their roots as an accounting firm.
Headquartered in Armonk, New York, IBM backs up their technological consulting with a robust suite of computing-related products and services. Engagements can range from application management to business analytics to developing and rolling out a customized solution enterprise-wide.
16) Ernst & Young
Headquartered in London, E&Y's consulting function provides advisory and insurance services for clients across all industries. A Big Four audit firm, E&Y has particular expertise in operational risk and compliance issues.
17) Towers Watson
Headquartered in Stamford, Connecticut, Towers Watson (formed by the 2010 merger of Towers Perrin and Watson Wyatt Worldwide) provides human resources, risk management, and reinsurance consulting from its 72 offices worldwide.
Headquartered in Amstelveen, the Netherlands, KPMG's management consulting group tackles four key areas: Business Effectiveness, Financial Management, People and Change, and IT. KPMG is primarily a tax and audit firm, and another set of services between accounting and traditional management consulting. These services include Risk Consulting and Transactions & Restructuring.
Headquartered in New York, Mercer is another member of the Marsh & McLennan family. They have over 19,000 employees operating in 40 cities across the world. Mercer focuses on human resources issues such as compensation, benefits, communication, and change management.
20) Aon Hewitt
Headquartered in Lincolnshire, Illinois, Hewitt is a leader in benefits consulting. Previously independenty operated, Hewitt was acquired by Aon in July 2010. Aon Hewitt consulting also works on engagements related to talent management, communications, and the people side of M&A.
21) Huron Consulting Group
With 14 offices growing out of Chicago, Huron assists a variety of clients but has some specialization within education, healthcare, and life sciences. Huron's functional specialties include litigation, restructuring, and turnaround.
22) Alvarez & Marsal
When companies get in a real mess, New York-based Alvarez & Marsal's professionals from 41 offices often parachute in to provide corporate turnaround and restructuring services. Sometimes Alvarez and Marsal provide detailed forensic accounting services to enable companies to re-list on stock exchanges. Other times, A&M will provide a CFO on an interim basis to straighten things out on the ground.
23) Hay Group
Headquartered in Philadelphia, the Hay Group's teams from 86 offices provide a wide array of solutions to human resource challenges, from retention to training to motivation.
Headquartered in Paris, Altran is a leader in technology and high technology innovation consulting throughout Europe. They have approximately 15,000 consultants operating within 20 countries.
25) PA consulting
Headquartered in London, PA operates in 35 countries with specialization in IT and innovation. They serve a wide variety of clients, including governmental agencies in areas such as defense and environmental sustainability.