 In philosophy, “self-knowledge” standardly refers to knowledge of one’s own mental states—that is, of what one is feeling or thinking, or what one believes or desires. At least since Descartes, most philosophers have believed that self-knowledge differs markedly from our knowledge of the external world (where this includes our knowledge of others’ mental states). But there is little agreement about what precisely distinguishes self-knowledge from knowledge in other realms. Partly because of this disagreement, philosophers have endorsed competing accounts of how we achieve self-knowledge and of its epistemic status. These accounts have important consequences for a broad range of issues in epistemology, the philosophy of mind, and moral psychology.