In this thesis I provide an interpretation of Kant's theories of knowledge, nature, and being in order to argue that Kant's ontology is a productive ontology: it is a theory of being that includes a notion of production. I aim to show that Kant's epistemology and philosophy of nature are based on a theory of being as productivity. The thesis contributes to knowledge in that it considers in detail Kant's ontology and theory of being, topics which have generally been ignored (...) or misunderstood. In arguing for Kant's productive ontology, I argue against Heidegger's interpretation of Kant, which states that Kant understands being as "produced permanent presence" or as divinely created materiality. Based on Kant's definition of being as positing, I argue, by contrast to Heidegger, that Kant understands being as the original productive relation between subject and object. This can also be expressed as the relation between formality and materiality, or between epistemic conditions and existence, that is productive of objects of experience. Being is not producedness but a relation of productivity, through which both subject and object are themselves productive. The subject is productive in its spontaneity, and nature, determined as dynamical interaction, is interpreted as productive. The subject, I will argue, does not understand nature as produced, but approaches it with a comportment towards its production as object of experience. Because of its own subjective productivity - spontaneity or "life" - the subject has a "productive comportment" towards nature. Ontology, I claim, concerns the realm of the productive relation of being, the realm of the relation between epistemic conditions and existence, and therefore the realm of possible experience. This marks Kant as divergent not only from what Heidegger calls "the ontology of the extant", but also from the concept-based ontology of the German rationalists. The general aims of the thesis are, first, to argue that being for Kant is the original relation between subject and object, and that ontology concerns this relation; second, to argue that ontology and being are understood in terms of production and productivity; and third, to argue that Heidegger is wrong to ascribe to Kant an understanding of being as "produced pennanent presence". I approach these aims by examining a number of Kant's texts in detail, focusing particularly on Kant's theses about existence and being in The One Possible Basis for a Demonstration of the Existence of God and the Critique of Pure Reason; on Kant's philosophy of nature and dynamical matter in the Transcendental Analytic and Metaphysical Foundations of Natural Science; on Kant's doctrine of experience and objectivity in the Transcendental Deductions; on ontological reflection and the productive comportment of "life" in the Critique of Judgment; and on Kant's final theory of matter, life and production in the Opus Postumum. (shrink)