 The idea of possible worlds is evocative and appealing. However,  possible worlds failed to gain any real traction among philosophers  until the 1960s when they were invoked to provide the conceptual  underpinnings of some powerful developments in modal logic. Only then did questions of their nature become a matter of the highest  philosophical importance. Accordingly, Part 1 of this article will provide an overview of the role of possible worlds in the  development of modal logic. Part 2 explores three prominent  philosophical approaches to the nature of possible   worlds.[1]  Although many of the finer philosophical points of Part 2 do presuppose the technical background of Part 1, the general philosophical landscape laid out in Part 2 can be appreciated  independently of Part 1.