This chapter defends a connection between knowledge and practical reasoning, according to which one’s reasons for action constitute all and only that which one knows. A variety of intuitive objections to such principles are considered and rejected—a central theme is that objectors to knowledge norms often make tacit but substantive ethical assumptions about which reasons, if held, would justify which actions. Absent broader ethical theorizing, the proposed counterexamples are inconclusive. The chapter sketches possible approaches to such theories, and indicates reason for optimism about knowledge norms. It also considers the degree to which knowledge norms imply externalism about rational action, suggesting that many internalist intuitions and verdicts may be accommodated and explained by knowledge norms.