This chapter defines job performance as the total expected value to the organization of discrete behaviors that an individual carries out over a standard period of time. Several taxonomic performance models in the literature describe categories of behavior believed to have organizational value. The categories have been defined either according to their manifest behavioral content (Campbell, 1990), their organizationally relevant consequences (Borman & Motowidlo, 1993), their motivational antecedents (Organ, 1988; Sackett, in press), or other antecedents such as ability and personality traits (Viswesvaran, Schmidt, & Ones, 2000). Performance antecedents include both direct determinants, such as knowledge, skill, motivation, habits, and situational opportunities and constraints, and indirect determinants, including individual differences in ability and personality and some types of situational variables. The direct determinants are presumed to mediate effects of the indirect determinants on job performance through causal mechanisms that involve capacity to learn, opportunity to learn, motivation to learn, and dispositional fit. An important implication of the performance definition presented in this chapter, taxonomic structures of behavioral dimensions of job performance, and its direct and indirect antecedents is that different traits, learning processes, motivational mechanisms, and situational constraints might have different effects on different behavioral dimensions of job performance.