The distinction between technical and contextual performance is widely recognized in the Industrial/Organization Psychology literature (Sackett & Lievens, 2008). Less well-understood are the causal antecedents of performance in these domains and how those antecedents relate to each other. Motowidlo, Borman, and Schmit (1997) proposed that technical performance is determined largely by cognitive ability, which acts through the mediator technical knowledge to influence technical performance. They also proposed that contextual performance is mainly determined by personality traits and that these traits influence contextual performance via the mediating variable contextual knowledge. Although prior research has examined some of the causal antecedents proposed by Motowidlo et al. (1997), no study has examined these four variables simultaneously, in addition to gathering information about performance criteria in the two domains. This study examined these six variables in a sample of medical students. In keeping with the verbiage used in the medical literature, students' contextual knowledge is referred to as professional knowledge and their contextual performance is referred to as professional performance. Medical students (N = 209) beginning their third year at the University of Texas Medical School at Houston completed measures of professional knowledge and the Big Five personality traits and consented to have their MCAT scores (a proxy for cognitive ability) and their first- and second-year course grades (grade point average; a measure of their technical knowledge) gathered for this investigation. Performance criteria consisted of attending physicians' ratings of students' professional and technical performance during their clinical rotations. Rotations were grouped according to whether they fell into the domain of Primary Care or the Specialties. Notable findings are summarized by a path analytic model. Agreeableness exerted a causal influence on professional knowledge (β = .38) and Primary Care professional performance (β = .14). Extraversion causally affected professional knowledge (β = -.22). Professional knowledge accounted for variance in Primary Care professional (β = .19) and technical performance (β = .22). Openness to experience and conscientiousness influenced technical knowledge (β's -.19 and .25). Cognitive ability was directly related to technical knowledge (β = .43) and Specialties professional (β = -.21) and technical performance (β = -.19). Technical knowledge was related to Primary Care professional (β = .32) and technical performance (β = .42) and also Specialties professional (β = .46) and technical performance (β = .57). Results generally suggest that separate causal paths underlie performance in Primary Care and the Specialties, respectively.