Multi-trait theories are designed to describe the entire personality. Their purpose is to locate the constellation of traits that make up the structure of personality and to devise appropriate measures of each of these traits. This approach assumes that we all share the same personality structure, although people differ from each other because we are each characterised by our own particular combination of trait scores. Hence the multi-trait approach captures both the underlying similarity between human beings and the surface differences; or, to use Allport’s terminology, it is able to make both nomothetic and idiographic statements about personality (Allport, 1937). It is surprising that, although the subject matter of the various multi-trait theorists has been the same (they have all been studying personality), they have nevertheless produced different models of personality structure. These differences, and the possible reasons for them, will be one of the main discussion points of this chapter.