Deloitte Touche Tohmatsu Limited (), commonly referred to as Deloitte, is a British multinational professional services network. Deloitte is the largest professional services network by revenue and number of employees in the world and is considered one of the Big Four accounting firms, along with EY, KPMG, and PwC.

The firm was founded by William Welch Deloitte in London, England in 1845 and expanded into the United States in 1890. It merged with Haskins & Sells to form Deloitte Haskins & Sells in 1972 and with Touche Ross in the US to form Deloitte & Touche in 1989. In 1993, the international firm was renamed Deloitte Touche Tohmatsu, later abbreviated to Deloitte. In 2002, Arthur Andersen's practice in the UK as well as several of that firm's practices in Europe and North and South America agreed to merge with Deloitte. Subsequent acquisitions have included Monitor Group, a large strategy consulting business, in January 2013. The international firm is a UK private company, limited by guarantee, supported by a network of independent legal entities.

Deloitte provides audit, consulting, financial advisory, risk advisory, tax, and legal services with approximately 457,000 employees globally, and operates in over 150 countries. In FY 2023, the network earned revenues of US$64.9 billion in aggregate. The firm has sponsored a number of activities and events including the 2012 Summer Olympics.

There have been a number of controversies surrounding the firm. These include Deloitte's involvement in a "potentially misleading" report on illicit tobacco trading in Australia. The company suffered a major cyber-attack in September 2017, which caused a breach in client confidentiality and made public a significant amount of employee information. Deloitte has been subject to litigation in relation to several audits it was responsible for. It was the internal auditor of the insolvent contractor Carillion and it was the external auditor of Autonomy, when accusations were made relating to "accounting improprieties". This included an £8.8 billion 'write-down' in its value, following Autonomy's acquisition by Hewlett-Packard in 2011. Provided by Wikipedia