Participation in high quality out-of-school-time activities constitutes a significant portion of the time that many youth spend away from their families or school settings, and current theory and research suggests that activity participation can be an influential contextual asset for promoting adaptive outcomes for youth. Therefore, the purpose of this chapter is to highlight how the relational developmental-systems-based positive youth development perspective is a useful framework for examining how and why high quality activity participation may be associated with positive developmental outcomes. As an example of research within this framework, we present findings from the 4-H Study of Positive Youth Development in order to illustrate how activity participation is an important facet of aligning individual youth strengths with resources within the environment. Finally, we discuss how to synthesize the research that exists on activity participation, and what the current research suggests for future empirical and applied steps in the field.