Interpersonal coaching behaviors play a central role in shaping youth players’ sport experiences, motivation, and well-being. Theory-informed coach development programs (CDPs), particularly those grounded in self-determination theory (SDT), have been proposed to promote supportive coaching environments. However, limited evidence exists on how such programs are delivered in real-world sport systems, how coaches and educators experience the implementation, and the extent to which they produce meaningful outcomes for coaches and youth players.

     The overall aim of this thesis was to investigate the delivery and effectiveness of interpersonal CDPs across formal and non-formal settings, and their effects on coach and youth player outcomes. The thesis comprises four individual studies.

     Study I was a systematic literature review and meta-analysis examining the effects of interpersonal CDPs and parent development programs (PDPs) on youth player outcomes. Results indicated small-to-medium increases in player enjoyment, self-esteem, and perceptions of coach competency and a mastery-oriented climate, while reducing anxiety. Study II employed a cluster-randomized controlled trial to evaluate an SDT-based coach education course delivered by the Swedish Ice Hockey Association. Compared with controls, intervention coaches reported increased competence-supportive behaviors over a three-week period. Study III used a mixed-methods design to examine the delivery of the same course. Quantitative data indicated high educator adherence and coach engagement, while qualitative interviews identified key barriers (e.g., time constraints, variation in content comprehension) and enablers (e.g., peer learning, flexible pedagogy, and educators’ experiential knowledge). Study IV evaluated a club-level SDT-based CDP using a three-arm quasi-experimental design. A workshop-only condition and a workshop plus smartphone-delivered booster condition (informed by just-in-time adaptive intervention principles) were compared with a control group. Over ten weeks, ice hockey players of both intervention groups reported increased relatedness-supportive coaching, while the workshop-only group also showed gains in autonomy support and autonomy satisfaction.

     Overall, the findings of this thesis support the effectiveness of interpersonal CDPs, including those grounded in SDT and related theoretically informed approaches, as a foundation across educational formats, and highlight the importance of implementation processes, contextual adaptation, and system-level support. Collectively, its findings provide conceptual, empirical, and methodological contributions to coach education policy and practice. It emphasizes that strengthening coaches’ interpersonal competencies can foster environments in which young people feel supported, motivated, socially connected, and able to develop a healthy and sustained relationship with sport.