According to a widespread interpretation of Berkeley’s philosophy, advocated, for example, by Kant and Reid, Berkeley’s main claim in the Treatise Concerning the Principles of Human Knowledge is that there are no material, but only mental entities. In the following essay, it is argued that this reading of Berkeley’s idealism is mistaken. Berkeley does not hold ontological idealism, that is, the view that there is not a material world, to be true, but only counterfactual idealism, that is, the claim that material entities counterfactually depend upon mental substances for their existence. While defending this interpretation of Berkeley’s philosophy, I examine in some detail his theory of ideas and his account of corporeal substances. I conclude by discussing several objections to the reading of Berkeley’s idealism advanced in this essay.