The document discusses the Big Four accounting firms - PwC, Deloitte, EY, and KPMG. It provides details on their evolution over time from the Big Eight firms in the 20th century, mergers that resulted in the Big Six and Big Five, and eventually the Big Four today. Key information on each firm is given, including revenues, number of employees, services offered, and brief histories. The Big Four audit most major global companies and provide a variety of assurance, tax, consulting and advisory services worldwide.