The focal aim of this dissertation project centers on understanding the importance of some of the underlying factors responsible for the socialization of achievement motivation in youth sport and its affective outcomes. Furthermore, this dissertation project focuses on the specializing stage of development, more specifically, student-athletes (N = 78) attending a compulsory school with a sports profile.This dissertation project was guided by the theoretical frameworks provided by achievement goal theory (Nicholls, 1984, 1989), implicit theories of ability (Dweck, 2000), Ames’ (1992a, 1992b) motivational climate, Big-Fish-Little-Pond Effect (Marsh, 1984; Marsh & Parker, 1984) athletic burnout (Raedeke & Smith, 2001), and gender as a social institution (Lorber, 1994).In the first study, the aim was to analyze and problematize athletic ability longitudinally and with a gender perspective as it is perceived, discussed, and valued by student-athletes. A mixed method approach was used in this study consisting of quantitative analysis (multilevel modeling) of a three-year, six-wave data collection (N = 78). Furthermore, semi-structured interviews were conducted with 27 of the student-athletes. The two main results of this study were that entity beliefs increased, and incremental beliefs decreased during the three-year period, and that gender added a further understanding of the beliefs of student-athletes regarding athletic ability.The second study aimed to examine achievement goals in youth sport longitudinally and the within-person effects of perceived motivational climates by coaches, peers, and parents on achievement goal orientations. The student-athletes (N = 78) completed a multi-section questionnaire, six times over a three-year period, assessing the study variables and the multilevel modeling analysis revealed that both the task orientation and the ego orientation decreased for this age group over the threeyear period. Furthermore, the perceived task involving peer climate was significantly and positively related to task orientation, and perceived ego-involving coaching climate was significantly and positively related to ego orientation.In the third study, the aim was to examine the developmental trajectories of student-athlete burnout perceptions and the within-person relationship between achievement goals and burnout perceptions. The participants (N = 78), time frame, and measurement points were the same in this study, as in studies I and II. The results from the multilevel growth models revealed that burnout perceptions increased for this age group over the three-year period. Furthermore, task orientation was significantly and negatively related to a reduced sense of accomplishment and sport devaluation.The findings from this dissertation project highlight some of the complexity of achievement motivation in youth sport; the relationships between this type of motivation and the context, in this case, a school with a sports profile, and organized sports, and significant others such as coaches, peers, and parents. Furthermore, the results from this dissertation project underline the advantage of considering a specific developmental stage when studying achievement motivation in youth sport longitudinally.