Throughout his ten-year term as United Nations Secretary-General, Kofi Annan pursued reform of the organisation in an effort to strengthen its legitimacy, effectiveness, efficiency, and fairness. This article begins with a definition of UN reform and identifies four different types: Security Council reform, reform of administrative practices, incremental procedural reforms, and creation of new institutions to address contemporary challenges. It next examines Kofi Annan’s initiation of reforms over the course of his term in office, from the time of his selection in 1996 to the end of his term in 2006. In addition to his engagement with all four forms, he also pursued improvement of the organisation in each of the three pillars of the UN Charter: security, development, and human rights, arguing that they were fundamentally interrelated. Kofi Annan believed that reform was a process, and he pursued it not only throughout his term in office but also afterwards through his foundation and other activities. His legacy of reform has been continued by his successors in the Office of Secretary-General.