Through secondary data analysis of a nationally representative sample, the study explored the relations between adolescent summer arrangements and associated developmental outcomes (well-being, externalizing behavior, body mass index, and academic performance). The study also examined if these relations were mediated by parental knowledge of youth whereabouts and activities, as well as if these relations differed for subgroups of youth regularly involved in self-care, parent care, or other adult care during the summer. Controlling for individual and demographic characteristics and prior measures of outcomes, summer organized activity participation was related to higher emotional well-being, academic performance and lower externalizing behavior and body mass index the following school year. Moreover, the relation between summer organized activity participation and adolescent outcomes was mediated by high levels of parental knowledge for emotional well-being, reading, math, and externalizing behavior, but not obesity. (Contains 4 tables, 1 figure, and 2 footnotes.)