This dissertation is a systematic account of the concept of modality in Leibniz’s thought. In chapter 1 I will give a detailed account of the metaphysics of Leibnizian possible worlds and show how to link them up with the notion of a possible world as it functions in contemporary possible worlds semantics. I then conclude with some observations on what it means to have a possible worlds semantics and give my solution to a standard problem with Leibniz’s infinite analysis approach to modality. In chapter 2 I show the development of and eventual deep connection in Leibniz thought between the infinite analysis and possible worlds accounts. Possible worlds supply the reason for the infinity of the analysis involved in contingent predicates. Chapter 3 begins with a summary of attempts in the literature to avoid the scope of Leibniz’s essentialism. I then argue that Leibniz employed an intensional theory of reference and provide a model for it. I conclude with an account of Leibniz’s treatment of counterfactuals. Lastly, in chapter 4 I give a treatment of Leibniz’s per se and moral modality. I conclude by showing in what sense Leibniz thought all of an individual’s properties are intrinsic and in what sense he didn’t think all of an individual’s properties were essential.