Foreign policy analysis benefits from careful attention to state identity. After all, identity defines the field itself by making it possible to speak both of policies and of a domain that is foreign. For some scholars, identity has proven useful as a guide to agency and, in particular, to agent preferences. For others, identity has served as a guide to social or institutional structure. Theories of state identity can be divided into three categories: conditions internal to agents, social interactions among agents, and “ecological” encounters with a broader environment. Internal conditions refer to either processes or constraints that operate within the agent under consideration. In the case of the state, these may include domestic politics, the individual characteristics of citizens or other internal actors, and the collective attributes of these citizens or other actors. Although internal causes are not social at the state level, they nevertheless have social implications if they give rise to state identity, and they may themselves be social at a lower level. The social interactions of states themselves constitute a second source of identity, one that treats states as capable of interacting like persons. This approach essentially writes large social and psychological theories, replacing individuals with the state. Finally, the ecological setting or broader environment is a third possible source of identity. The environment may be material, ideational, or discursive, and treated as an objective or a subjective influence.