This is a work in analytic metaphysics. Its main purpose is to clarify a notion of central importance in metaphysics since Aristotle, to wit the notion of existential dependence. All currently available analyses of the notion are examined and then rejected, and a new account is defended. This work is the first comprehensive one on the topic. The first chapter is devoted to introducing and explaining some notions which are crucial for the central parts of the work, namely the notions of existence, necessity, (individual and plural) quantifi- cation and essence. In chapters 2 and 4 focus is made on the relation of simple existential dependence, the relation which holds between two objects when the first cannot exist without the other. Three accounts of simple dependence - each endorsed by some contemporary philosophers, among them Kit Fine, E. Jonathan Lowe, Kevin Mulligan, Peter Simons and Barry Smith - are presented and then rejected. A new account, inspired by suggestions by Fine and Lowe, is defended. According to that account - the foundational account - simple dependence is to be defined in terms of a relation called 'grounding', which is presented in chapter 3. Chapters 5 and 6 deal with relations belonging to the family of simple dependence, among others (i) generic dependence, (ii) various forms of temporal dependence, and (iii) supervenience, a complex dependence relation largely invoked in current debates on the philosophy of mind. It is shown that foundationalist accounts of these notions - i.e. accounts framed in terms of grounding are superior to other existing accounts. These chapters also contain some applications of the foundational conception of dependence, in particular a characterization of substances and a formulation of the distinction between two well known conceptions of universals, the Aristotelian and the Platonician conception. The last part of the work is a technical appendix where one can find, among other things, a system for the logic of essence, which is proved to be sound and complete with respect to a possible world semantics. The Author: Born 1972. PhD in Philosophy, University of Geneva (2002). Now appointed professor of analytic philosophy at the Department of Philosophy of the University of Geneva, and in charge, with Kevin Mulligan, of the research group in metaphysics eidos. Correias philosophical interests are in metaphysics, philosophical logic, the philosophy of language, the philosophy of science, epistemology and the philosophy of mind.