Possibilities divide into two kinds. Non-qualitative possibilities are distinguished by their connection to specific individuals. For example, the possibility that Napoleon is a novelist is non-qualitative, since it is a possibility for a specific individual, Napoleon. In contrast, the possibility that someone---anyone at all---is a novelist is a qualitative possibility, since it does not depend upon any specific individual. Haecceitism is a thesis about the relation between qualitative and non-qualitative possibilities. In one guise, it holds that some maximal possibilities---total ways the world could be---differ non-qualitatively without differing qualitatively. It would, for example, be only a haecceitistic difference that distinguishes actuality from a maximal possibility where Napoleon and Nefertiti swap all of their qualitative properties and relations. According to this alternative possibility, things are the very same qualitatively, but which individuals occupy which qualitative roles differs: Nefertiti would be a stout conqueror, while Napoleon would be a beautiful consort. This dissertation is an examination of the nature of haecceitism, the arguments in its favor, and the consequences that follow from it. In Chapter One, I distinguish various conceptions of haecceitism and related theses concerning maximal possibilities, possible worlds, the identity of indiscernibles, and non-qualitative properties. In Chapter Two, I develop and defend conceivability arguments for haecceitism in the face of various anti-haecceitist challenges. In Chapter Three, I consider the relation between haecceitism and the Humean approach to plenitude, which aims to characterize the space of possible worlds in terms of combinatorial principles. In Chapter Four, I examine the distinction between qualitative properties like redness and non-qualitative properties like being Napoleon and argue in favor of fundamental non-qualitative properties. In Chapter Five, I present a novel version of non-qualitative counterpart theory, which employs bare particulars to reconcile modal realism and haecceitism. In Chapter Six, I clarify and defend quidditism, the property-theoretic analogue of haecceitism. I conclude in Chapter Seven by defending the modal view of essence.