Deloitte

William Welch Deloitte opened an accounting practice in London in 1845, at a time when the profession of accountancy barely existed as a formal discipline. His early clients included the Great Western Railway — one of the most ambitious infrastructure projects in Victorian England. The railways were the first businesses complex enough to require independent financial oversight. In 1849, Deloitte was appointed the first independent auditor of a public company in history.

Deloitte opened its first U.S. office in New York in 1893, following its British clients as they expanded into American markets. The U.S. would eventually become the firm's largest market. The transatlantic expansion established a template that the major accounting firms would follow for the next century: grow internationally by following clients, not by seeking new ones.

Deloitte Haskins & Sells merged with Touche Ross in 1989, creating Deloitte & Touche — one of the largest accounting mergers in history. The combined firm had over 60,000 employees in 100 countries. The merger was driven by the need to serve multinational clients who required consistent audit standards across borders. This consolidation dynamic would eventually reduce the global audit market to just four dominant firms.

When Arthur Andersen collapsed in 2002 following the Enron scandal, Deloitte was the only Big Five firm that did not aggressively acquire Andersen's practice. While KPMG, Ernst & Young, and Pwcabsorbed thousands of Andersen partners and clients, Deloitte grew more selectively. The restraint proved wise: the firms that grew fastest from Andersen's collapse also inherited the most regulatory scrutiny.

Deloitte reported revenues of approximately $64 billion in fiscal year 2023, making it the world's largest professional services firm — a position it retained for years afterward, ranked No. 1 by revenue by Gartner for the eighth consecutive year in 2025. The firm employed over 450,000 people across 150 countries. However, the consulting industry faced headwinds in 2024-2025: Deloitte cut advisory roles in the UK as large-scale project demand softened, and reduced U.S. government consulting headcount as federal contracts were restructured under the DOGE efficiency initiative. AI was simultaneously threatening to automate the analytical work that had made consulting firms valuable — and creating enormous new demand for AI strategy and implementation work. Deloitte was racing to position itself on both sides of that disruption.