Any inquiry into identity disorders faces the difficulty that the ordinary understanding of personal identity is itself ambiguous and contentious. In what follows, the concept of personal identity that has been of principal philosophical interest is distinguished and clarified, and ideas about the nature of the self are reviewed. The most influential approach to persons and their identity, deriving from the work of John Locke, is then set out as a basis for reflection on disorders. Varying degrees of disruption to the unity of consciousness are then considered, together with the effect of these on the conception of the self and its continuing identity. Finally, there is a discussion of dissociative identity disorder and of the way in which its conceptualization relates to its status as a disorder.