Many organized out-of-school-time (OST) activities provide enriching opportunities for children to interact with peers, build cognitive skills, develop relationships with mentors, and explore a variety of talents and leisure pursuits. A growing set of findings suggests that youth participation in OST activities is associated with academic achievement and overall well-being. Adolescents who participate in these activities have lower school absenteeism, are less likely to drop out, like school more, get better grades, and are more likely to attend college than adolescents who do not participate in these activities. Activity participation is also associated with positive social adjustment, such as low problem behavior, drug use, and loneliness. This growing literature has greatly added to the people's knowledge concerning the role of OST activities in children's lives. Yet several questions remain. First, most research on this topic has been conducted with adolescents, but OST activities are an important component of many children's lives during the elementary school years as well. In this article, the authors use data from two samples to examine whether there are gender and sample differences in participation in various organized activities and whether the relations of participation to adjustment are stronger for some subgroups than for others. Although children in both samples are of roughly the same age and report on similar measures of participation and outcomes, their family incomes, socioeconomic status, ethnicity, and neighborhoods are very different. The youth in the first study, Childhood and Beyond (CAB), come from largely white, middle-class families in urban, suburban, and rural Michigan, whereas those in the second study, New Hope, are from low-income families in Milwaukee, who are primarily African American and Hispanic. (Contains 2 figures and 25 notes.)