What is separation of duties and why is it a critical control?

Separation of duties divides critical tasks among multiple people so that no single individual can complete a high-risk process alone, preventing fraud and errors. For example, the person who requests a purchase should not be the same person who approves payment. In IT, the developer who writes code should not be the same person who deploys it to production. This control requires collusion between multiple individuals to commit fraud, making it significantly more difficult.