The prevalence of interpersonal conflict within sport relationships is high (Mellalieu et al., 2013) and seems to be posing a threat to effectiveness of coach-athlete interactions. However, despite the importance of these working alliances for sport performance and wellbeing, only little research efforts have been directed towards interpersonal processes that may impair the quality of coach-athlete relationship (e.g., disagreements, dispute). Therefore, the purpose of this PhD research project was to develop a fundamental understanding of interpersonal conflict between coaches and their athletes. The presented studies built upon a scoping review (Wachsmuth, Jowett, & Harwood, 2017) in which an initial conceptual framework and a definition of interpersonal conflict within sport relationships was forwarded. Thus, based on the available literature interpersonal conflict was defined as “a situation in which relationship partners perceive a disagreement about, for example, values, needs, opinions, or objectives that is manifested through negative cognitive, affective, and behavioural reactions” (p. 87).