Big Four accounting firms

The Big Four are the four largest professional services networks in the world: Deloitte, EY, KPMG, and PwC. The four are grouped because of their large sizes relative to their competitors, both in terms of revenue and workforce headcount; they are likewise considered comparable in their ability to provide consistent and reliable professional services to their clients. Colloquially, their networks are considered of parity to those seeking careers in the services offered by these firms because of the frequency with which they engage with Fortune 500 companies.

The Big Four offer audit, assurance, taxation, management consulting, valuation, market research, actuarial, corporate finance, and legal services to their clients. A significant majority of the audits of public companies, as well as many audits of private companies, are conducted by these four networks. Until the late 20th century, the market for these professional services was dominated by eight firms, which were accordingly nicknamed the Big Eight. The Big Eight consisted of Arthur Andersen, Arthur Young, Coopers & Lybrand, Deloitte Haskins and Sells, Ernst & Whinney, Peat Marwick Mitchell, Price Waterhouse, and Touche Ross.

The Big Eight gradually evolved into the Big Four due to mergers between some of these firms, and culminated in the 2002 collapse of Arthur Andersen, leaving four networks dominating the market at the turn of the 21st century. In the United Kingdom in 2011, it was reported that the Big Four account for the audits of 99% of the companies in the FTSE 100 Index, and 96% of the companies in the FTSE 250 Index, an index of the leading mid-cap listing companies. This high level of industry concentration has caused concern among critics, and is the chief reason some notable people of business have expressed the desire for government bodies such as the UK's Competition & Markets Authority (CMA) to consider breaking up the Big Four.