List the Big Four accounting firms.

None of the "firms" within the Big Four is actually a single firm; rather, they are professional services networks. Each is a network of firms, owned and managed independently, which have entered into agreements with the other member firms in the network to share a common name, brand, intellectual property, and quality standards. Each network has established a global entity to co-ordinate the activities of the network.

Until 2020, KPMG[3] was the only Big Four firm not registered as a UK private company, but rather the co-ordinating entity was a Swiss association (verein). However, KPMG International changed its legal structure from a verein to a co-operative under Swiss law in 2003,[4] then to a UK limited company in 2020.[3] For Deloitte,[5] PricewaterhouseCoopers[6] and Ernst & Young,[7] the co-ordinating entity is a UK limited company. Those entities do not themselves perform external professional services, nor do they own or control the member firms. Nevertheless, these networks colloquially are referred to as "firms" for the sake of simplicity and to reduce confusion with lay-people. These accounting and professional services networks are similar in nature to how law firm networks in the legal profession work.

In many cases, each member firm practices in a single country, and is structured to comply with the regulatory environment in that country.

Ernst & Young also includes separate legal entities which manage three of its four geographic areas: the Americas, Asia-Pacific, and EMEIA (Europe, the Middle East, India and Africa) groups, the fourth area being Japan, which has no larger co-ordination branch. These entities coordinate services performed by local firms within their respective areas, but do not perform services or hold ownership in the local entities.[8] There are rare exceptions to this convention; in 2007, KPMG announced a merger of four internationally distinct member firms (in the United Kingdom, Germany, Switzerland and Liechtenstein) to form a single firm, KPMG Europe LLP.[9]

The Big Four refers to the the four largest professional services networks in the world. It consists of 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.