The notion of personality traits has received widespread acceptance in light of the universal consistencies shown in individuals' behaviours and responsivities to situational stimuli. Trait theory was promoted strongly by pioneering personologists such as Gordon Allport, Raymond Cattell, and Hans Eysenck. Indeed, in terms of citations in the peer-reviewed journal literature, both Cattell and Eysenck were listed among the top 10 most highly cited psychologists of the 20th Century (Haggbloom et al., 2002, p. 142). Debates about factor analytic methodology have often served to obscure the fact that both Cattell and Eysenck were in much agreement in relation to their taxonomic findings into human personality structure. Eysenck (1984, p. 336) even acknowledged that, "The Cattell and Eysenck constructs and theories should be seen, not as mutually contradictory, but as complementary and mutually supportive." More recently, the Five Factor Model (FFM) has become prominent as a putative framework for organising personality trait data (see McCrae & Costa, 2008). Although the FFM has generated much empirical data, substantive objections to the FFM have been raised in relation both to the validity of dimensional models generally (e.g., McAdams, 1992), and to the psychometric evidence more specifically (Block, 1995; Boyle, 2008). Nevertheless, progress in understanding traits has been evidenced by three important advances: 1) a more sophisticated understanding of the biological bases of traits. 2) an increased integration of trait research within mainstream cognitive, social and developmental psychology. 3) an increased focus on assessing traits. In terms of personality assessment, the major focus to-date has been on introspective (i.e., subjective) self-report questionnaires and rating scales. There are signs, however, that research into the construction of computer-interactive objective personality tests will become more prominent during the 21st Century.