More and more philosophers are using modal concepts to shed light on various philosophical problems and to analyse philosophical concepts. This introduction to the topic of modality emphasizes the metaphysics of modality - in particular, the view that a commitment to possible worlds gives us the best way to understand modality - rather than the formal semantics of quantified modal logic. Joseph Melia shows how different theories about possible worlds not only influence our more general modal beliefs but illustrate and illuminate methodological considerations such as ontological and ideological parsimony, the degree to which any philosophical theory ought to respect common sense, and the nature of philosophical analysis. Melia begins by introducing students to the de re/de dicto distinction, conventionalist and conceptualist theories, and some of the key problems in modality. He then explains how possible worlds provide a solution to many of these problems and how possible worlds themselves have been used to analyse notions outside modality such as properties and propositions. Melia also shows how possible worlds can introduce new problems of their own and argues that to make progress with these problems a theory of possible worlds is required. The pros and cons of various theories are examined in turn, including those of Lewis, Kripke, Adams, Stalnaker, and Plantinga.