The essence-existence distinction was a central issue in metaphysical disputes among post-Avicennian thinkers in the Islamic world. One group argued that what a thing is is different from that it is only conceptually. A rival view would have it that the distinction between essence and existence is real. The purpose of this article is to analyze the philosophical core of the dispute, by isolating the main arguments and their metaphysical foundations. I will study four central issues of the essence-existence debate: (1) the argument that existence is distinct from essence because one can conceive of an essence without knowing whether it exists; (2) the argument that if existence were really distinct from essence, existence would itself have to exist, leading to an infinite regress; (3) the question of whether God is responsible for the existence of essences only or also for their essential content (this relates to the problem of the ontological status of the non-existent); (4) the problem of whether essences are prior to existence.