In this paper I reconstruct Hans Kelsen's doctrine of legality. By legality I mean here, loosely speaking, the requirement that public authority must be exercised in accordance with the law. My reconstruction of Kelsen’s doctrine is guided by two related questions: 1) what does legality require? and 2) how can this effectively be realized? For Kelsen, the principle of legality forbids acts and norms that are not authorized by the objectively valid legal order. For the principle to become effective, it must be supplemented by practical guarantees of legality, including constitutional review.