Deloitte Touche Tohmatsu Limited ( /dəˈlɔɪt/), commonly referred to as Deloitte, is one of the Big Four professional services firms along with PricewaterhouseCoopers (PwC), Ernst & Young, and KPMG.

Deloitte is the second largest professional services network in the world by revenue and has 182,000 employees in more than 150 countries providing audit, tax, consulting, enterprise risk and financial advisory services. In FY 2011, Deloitte earned a record $28.8 billion USD in revenues, ranking second behind PwC's record $29.2 billion.

Its global headquarters is located in New York City, United States.

In 1845 William Welch Deloitte opened an office in Basinghall Street in London. Deloitte was the first person to be appointed an independent auditor of a public company. He went on to open an office in New York in 1880.

In 1896 Charles Waldo Haskins and Elijah Watt Sells formed Haskins & Sells in New York.

In 1898 George Touche established an office in London and then in 1900 joined John Ballantine Niven in establishing the firm of Touche Niven in the Johnston Building at 30 Broad Street in New York. At the time, there were fewer than 500 CPAs practicing in the United States, but the new era of income taxes was soon to generate enormous demand for accounting professionals.