This entry is a general overview of the issues raised by haecceitism and anti-haecceitism. In Sections One through Three, various formulations of haecceitism are presented and the connections between haecceitism, haecceities, and essentialism are examined. In Sections Four and Five, arguments for and against haecceitism are surveyed. Section Six discusses the scope of haecceitistic differences and the prospects for accepting only certain kinds of haecceitistic differences. Finally, Section Seven briefly notes the significance of haecceitism and its denial within various areas of metaphysics.