Deloitte Touche Tohmatsu Limited

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, and Deloitte was among the first to provide it. 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 — a demand that would eventually consolidate the global accounting industry into just four dominant firms.

When Arthur Andersen collapsed in 2002 following the Enron scandal, Deloitte was the only Big Five firm that did not acquire a significant portion of Andersen's practice. While KPMG, Ernst & Young, and PricewaterhouseCoopers absorbed 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 largest professional services firm in the world, ahead of PwC, EY, and KPMG. The firm employed over 450,000 people across 150 countries. Deloitte's consulting practice had grown to rival dedicated management consultancies like McKinsey and BCG, blurring the line between audit firm and strategy advisor that regulators had long sought to maintain.