This book offers an in-depth and updated examination of the nature of haecceity-that primitive entity which explains why something is distinct from other things. The book begins by exploring different conceptions of haecceity throughout history. The discussion of various figures across history is important for getting clear on the nature of haecceity and its role in individuation. The next part of the book examines different views about the nature of haecceity. The author defends a view on which haecceities have objects that instantiate them as constituents. Following that, the book considers arguments for and against the existence of haecceities, the epistemology of haecceity, and the distinction between qualitative and non-qualitative properties. About Haecceity will appeal to scholars and advanced students working in metaphysics, philosophy of language, epistemology, logic, and history of philosophy.