Research on coach-athlete relationships requires a sound theoretical foundation and very thoughtful selection of methods of inquiry. A serious limitation in prior research on coach-athlete relationships has been the sole focus on the coach’s interpersonal behaviors and their influence on a limited number of coach as well as athlete outcomes (e.g., basic needs satisfaction, behavioral regulation, and physical and psychological well-being). This, in turn, has created a ‘uni-directional’ bias in the sport and exercise psychology literature, thereby misrepresenting the complex issues that are inherently present in the interpersonal dynamics in the coach-athlete relationship. Hence, the purpose of this postdoctoral research proposal (henceforth called ‘the study’) is to extend prior work by taking into account the truly interpersonal nature of the coach-athlete relationship. Whereas social psychological concepts (e.g., person perception, communication, and leadership) typically involve two persons or groups of persons, previous research has mostly ignored the fact that observations from social data do not refer to a person, but instead refer to multiple persons embedded within the same social context. This is also true for research in sport and exercise psychology. Therefore, the current study will use methods (i.e., research designs and data analytic methods) that are appropriate for analyzing data in which observations (i.e., persons) within a group may influence one another. In addition, the findings will be discussed in relation to recent developments in the field of longitudinal research methods and motivation, and in terms of their implications for sport organizations and future research focusing on the social nature of the coach-athlete relationship.