Possible worlds have been the focus of great interest in metaphysics since the 1960s. This course will have three sections. In the first, we will examine some of the applications of possible world theory: to understanding modality; to accounts of mental content and the meaning of natural languages; to our understanding of conditionals; and applications in metaphysics including to theories of properties and chances. In the second, we will examine theories of the nature of possible worlds, and whether we should believe in possible worlds in the first place. In the third section, we will discuss de re modality and the so-called “problem of trans-world identity”. The kicking-off point for a lot of this course will be David Lewis’ classic On the Plurality of Worlds, but the course aims to be a general look at the philosophy of possible worlds and not just an examination of Lewis’ views.