Adolf Reinach made significant contributions to the early phenomenological movement with his work on states of affairs, the ontological categorization of the a priori, and a realist interpretation of essences, yet today his name and contributions go largely unrecognized. To make matters worse, the few who have contributed to Reinach scholarship have seriously misrepresented central features of his thought. This work seeks to rectify this situation by offering an historical account of the origin and development of some of Reinach's key notions and of their significance to the development of phenomenology in the early twentieth century. The intention of this book is thereby to reopen avenues of realist phenomenological inquiry that have been left unexplored for almost a century.