The notion of a theory of lineups is debatable and, in the minds of many psychological theorists, might seem peculiar. Psychological theories usually concern circumscribed psychological processes. For instance, there are theories of face recognition, theories of judgment and decision making, theories of attitude formation and change, and so on. In contrast, a lineup is a task or a procedure, and, although it involves psychological processes, it is not a psychological process per se. Nevertheless, we contend that it is possible to describe some necessary components of a theory of lineups. In particular, we propose various considerations that ought to be a part of an applied theory of lineups. We call it an applied theory because it is embedded in the ecology of the legal system and because it rests heavily on the applied goals of the users of lineups, principally the goal of incriminating the guilty and exculpating the innocent.