This paper moves from a disagreement with those interpreters who explain Kant’s doctrine of real possibility in terms of possible worlds. It seems to me that a possible world framework is too much indebted to the Leibnizian metaphysics of modality and, therefore, cannot serve to make sense of Kant’s theses. Leibniz’s theory of possibility, indeed, has been deeply criticized in Kant’s Critique of Pure Reason (CPR). Interestingly enough, however, Kant’s principal argument for rejecting that the field of what is possible is greater than the field of what is real was already anticipated by Leibniz. However, Leibniz employed it to demonstrate that there cannot be more than one actual world only (the others being purely possible ones). Moving from this fact, I argue that there is a certain tension between what Leibniz says about the actual world and his commitment to a plurality of possible worlds conceived as ideas in God’s mind. The first part of my paper is devoted to show that such a tension can be traced back to Leibniz’s claims about the relation between the possible and the real. In the second part, then, I maintain that Kant’s theory of real possibility grows from a dissatisfaction with (and a rejection of) Leibniz’s attempted solution to the problem of characterizing a kind of possibility narrower than the merely logical one and, nonetheless, not identical with existence. Finally, I present a short account of Kant’s theory of real possibility, based on the notion of transcendental conditions as conditions of possibility of experience, showing how it works in the case of the forms of intuition.