In this paper I explore Husserl’s and Foucault’s approaches to the historical a priori and defend Husserl’s richer notion. Foucault borrows the expression ‘historical a priori’ from Husserl and there are continuities, but also significant and ultimately irreconcilable differences, between their conceptions. Both are looking for ‘conditions of possibility,’ forms of ‘institution’ or instauration, and patterns of transformation (breakthroughs, disruptions), for scientific knowledge. Husserl identifies the ‘a priori of history’ with the ‘historical a priori’ and believes that the ‘invariant essential structures of the historical world’ (Crisis of European Sciences) can be identified. Foucault, on the other hand, is less interested in the Kantian inquiry into the limits or legitimization of knowledge than in the relation between knowledge and power. Foucault rejects the idea of universal and necessary a priori structures and denies that the structure of the conceptual framework (‘episteme’) governing an era can be fully determined. Both Foucault and Husserl contrast ‘inner’ history with external history, but, I argue, Foucault misconstrues Husserl’s transcendental phenomenology as a form of ‘absolute subjectivity’ against which his ‘archaeological’ method reacts. In fact, Foucault’s own conception of the historical a priori is ambiguous and fails to have explanatory value precisely because it misunderstands the need for the a priori to be both universal and necessary, and offers no account of the ‘a priori of historicity’ which, for Husserl, is essential to human cultural life.