This chapter explores the relation of identity, focusing on numerical identity—the relation that each entity holds with itself—and distinguishing it from qualitative identity. It introduces the standard unitary conception of identity, according to which identity is governed by reflexivity (every entity is identical to itself) and the indiscernibility of identicals (identical entities share all properties). The indiscernibility of identicals is a scarcely controversial metaphysical principle and should be distinguished from the principle of substitutivity of coreferential expressions, which concerns language and is affected by some tricky counterexamples. Challenges to this framework include attempts to reduce identity to indiscernibility and theories of relative identity, where identity is not absolute but contextual and dependent on sortals. The attempts to reduce identity to indiscernibility are at risk of circularity and assume the controversial principles of the identity of indiscernibles (entities sharing all properties are identical), for which there are possible counterexamples. Relative identity is a complex theory, which uses sortal concepts in order to distinguish different relations of identity and rejects the indiscernibility of identicals.