Although the philosophical systems of G. W. Leibniz and David Lewis both feature possible worlds, the ways in which their systems are similar and dissimilar are ultimately surprising. At first glance, Leibniz's modal metaphysics might strike us as one of the most contemporarily relevant aspects of his system. But I clarify in this paper major interpretive problems that result from understanding Leibniz's system in terms of contemporary views (like Lewis's, for instance). Specifically, I argue that Leibniz rejects the inference that if something is possible, it therefore occurs in some possible world. This discussion highlights how Leibniz's account of individual substance (with his strict notion of identity) constrains his modal theorizing and produces fatalistic threats. I then make an unexpected connection between Leibniz's and Lewis's systems by showing that Leibniz's treatment of fatalism bears similarities to the response Lewis gives to the fatalist when considering the paradoxes of time travel.