Thesis (Ph.D., Movement & Leisure Sciences) -- University of Idaho, 2016 | This dissertation is comprised of two studies based on data collected from NCAA Division I student-athletes comparing treatment and intervention groups during a 9-week career development intervention. Study 1 examined intervention effectiveness comparing psychosocial and behavioral outcomes over time, and Study 2 investigated why the intervention was effective by examining psychosocial and contextual factors as potential mediators and/or moderators of the relationship between the intervention and behavioral outcomes. Repeated-measures analysis of variance results from Study 1 indicated that the career development intervention significantly enhanced career decision-making self-efficacy, positive emotions, identified regulation, integrated regulation, self-reported career development behaviors, and stage of change; and significantly decreased amotivation towards career development. Results from Study 2 indicated that career decision-making self-efficacy, identified regulation, and integrated regulation significantly mediated the relationship between the intervention and several of the targeted psychosocial and behavioral outcomes. Intervention engagement was found to be a significant moderator of the relationship between the intervention and several targeted behavioral outcomes. The discussion focuses on the impact of the career development intervention on student-athletes and identifying the psychosocial and contextual factors that are critical to the effectiveness of the intervention. Suggestions are made for maximizing the effectiveness career development interventions targeted towards student-athletes and potential directions for future research on student-athlete career development interventions.