The Big Four are the four largest professional services networks in the world, the global accounting networks Deloitte, Ernst & Young (EY), KPMG, and PricewaterhouseCoopers (PwC). The four are often grouped because they are comparable in size relative to the rest of the market, both in terms of revenue and workforce; they are considered equal in their ability to provide a wide scope of professional services to their clients; and, among those looking to start a career in professional services, particularly accounting, they are considered equally attractive networks to work in, because of the frequency with which these firms engage with Fortune 500 companies.

The Big Four all offer audit, assurance, taxation, management consulting, 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 professional services was actually dominated by eight networks which were 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 reduced due to mergers between these firms, as well as 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. Such a high level of industry concentration has caused concern, and a desire among some in the investment community for the UK's Competition & Markets Authority (CMA) to consider breaking up the Big Four. In October 2018, the CMA announced it would launch a detailed study of the Big Four's dominance of the audit sector. In July 2020, the UK Financial Reporting Council told the Big Four that they must submit plans by October 2020 to separate their audit and consultancy operations by 2024.