W illiam "Bill" Ackman is a prominent and often controversial American billionaire who is one of the world's most famous activist hedge fund managers. He is the founder and CEO of Pershing Square Capital Management, a New York-based hedge fund. After an early career in real estate and at another hedge fund, he founded his own firm in 2004. He is known for his high-profile and often theatrical activist campaigns, where he takes large, concentrated positions in a small number of public companies and then publicly agitates for major changes in their strategy, management, or board of directors.

His career has been marked by both spectacular successes, such as his investments in Canadian Pacific Railway and Chipotle, and spectacular failures, most notably his disastrous bet against the nutrition company Herbalife and his ill-fated investment in the pharmaceutical company Valeant. He is also known for a single, brilliant trade at the outset of the COVID-19 pandemic, where he turned a $27 million investment in credit default swaps into a $2.6 billion profit for his fund. He and his wife are major philanthropists and signatories of the Giving Pledge.