The Big Four Auditors

"The Big Four
are the four largest international professional services networks in accountancy
and professional services,
which handle the vast majority of audits for publicly traded companies
as well as many private companies, creating an oligopoly in auditing large companies.

The Big Four firms ...latest publicly available data:

PwC (officially Price Waterhouse Coopers)
Revenues $29.2bn
Employees 169,000 2011

Deloitte Touche Tohmatsu
Revenues $28.8bn
Employees 182,000

Ernst & Young
Revenues $22.9bn
Employees 152,000

KPMG
Revenues $22.7bn
Employees 145,000

...The Big Five became the Big Four after the demise of Arthur Andersen in 2002,
following its involvement in the Enron scandal."

http://en.wikipedia.org/wiki/Big_Four_auditors