gathers together papers by an international group of academic experts, examining the metaphysical concept of substance in the writings of these two towering philosophers of the early modern period. Each of these newly-commissioned essays considers important interpretative issues concerning the role that the notion of substance plays in the work of Locke and Leibniz, and its intersection with other key issues, such as personal identity. Contributors also consider the relationship between the two philosophers and contemporaries such as Descartes and Hume.