The chapter introduces the iterative conception, according to which every set appears at one level or another of the mathematical structure known as the cumulative hierarchy, as well as theories based on the conception. The chapter presents various accounts of the iterative conception: the constructivist account, the dependency account and my own minimalist account. It is argued that the minimalist account is to be preferred to the others. A method – which I call inference to the best conception – is then described to defend the correctness of the iterative conception so understood. This method requires one to show that the iterative conception fares better than other conceptions with respect to a number of desiderata on conceptions of set. This provides additional motivation for exploring alternative conceptions of set in the remainder of the book.