This chapter analyzes the transformational journey that plans for the United Nations undertook from summer 1941 to the San Francisco Conference of 1945 at which the UN Charter was agreed. Prior to the conference, the ‘Big Three’ great powers of the day—the United States, the Soviet Union, and the United Kingdom—often struggled to establish the common ground on which the UN’s success would depend. However, their debates were only the start of the diplomatic travails which would eventually lead to the establishment of the world organization that we know today. Once gathered at San Francisco, the fifty delegations spent the next two months locked in debate over issues such as the role of international law; the relationship between the General Assembly and Security Council; the permanent members’ veto; and Charter amendment. One of modern history’s most important diplomatic events, its outcome continues to resonate through world politics.