This essay gives an interpretation of Heidegger's “What is Metaphysics?” lecture in light of passages from his other writings and lecture courses of the period. This exegetical task is important, for interpreters of “What is Metaphysics?” have been confused by puzzling phrases in the lecture without noticing that Heidegger makes the same points in clearer terms elsewhere. In particular, these interpreters ignore Heidegger's crucial distinction between entities and the being of entities. Since Heidegger's “nothing” is an aspect of being, this difference is at the core of Heidegger's lecture. The present interpretation establishes a conditional conclusion: If the ontological difference makes sense, then we have a sound basis for understanding “What is Metaphysics?” and do not need to read Heidegger as an irrationalist who debunks science or rejects the principle of contradiction. This paper does not give independent justification for the ontological difference.