This book presents a systematic discussion of the development of Husserl's concept of the a priori from his early and through his later writings.  The chapters contained herein analyze the different phases and aspects of Husserl's phenomenology of the a priori in light of his twofold notion of reason, construed as both ontological and transcendental. Starting from the assessment of the introduction of the notion of a priori knowledge in the context of the  Logical Investigations , this text uniquely explores its development during the Göttingen years. It is at this time during his work on The   Crisis of European Sciences , that Husserl comes to see the a priori as a criterion to interpret the history of philosophy, notably, modern philosophy. This book sheds light upon such concepts as: essence and  eidos ; ideation, eidetic attitude and eidetic reduction; as well as formal and material, innate and contingent a priori. The author argues that the a priori becomes for Husserl the expression of an ontological form of rationality, i.e., the rationality immanent to being. This book appeals to students and researchers working on Husserl and phenomenology.