There is increasing awareness that how young people spend their time outside of school has consequences for their development. As part of this awareness, interest in organized activities--extracurricular activities, after-school programs, and youth organizations--has grown markedly. On balance, the bulk of research on organized activities has shown positive consequences of participation for academic, educational, social, civic, and physical development. This fact, coupled with the safety and supervision provided by organized activities for youth with working parents, has fueled initiatives at the local, state, and Federal levels to expand opportunities for participation. In seeming opposition to these initiatives, there exists concern that participating in organized activities has become excessive for youth. This "over-scheduling" is thought to result from pressure from adults (parents, coaches, teachers) to achieve and attain long-term educational and career goals. These external pressures, along with the activity-related time commitment, are believed to contribute to poor psychosocial adjustment for youth and to undermine their relationships with parents. Because the implication is that increasing amounts of organized activity participation will be harmful to youth and family functioning, attention from scientists, practitioners, and policymakers is warranted. To evaluate these two somewhat different perspectives, we review two types of evidence: evidence from published studies focused on regional, historical, or limited samples and evidence from a very recent nationally representative sample of America's 5- to 18-year olds that includes both time use data and information on a wide range of indicators of development. The main findings across studies are as follows: (1) The primary motivations for participation in organized activities are intrinsic (e.g., excitement and enjoyment, to build competencies, and to affiliate with peers and activity leaders). Pressures from adults or educational/career goals are seldom given as reasons for participation; (2) American youth average about 5 hours/week participating in organized activities. At any given time, roughly 40% of young people in the US do not participate in organized activities and those who do typically spend less than 10 hours/week participating. Many alternative leisure activities (e.g., educational activities, playing games, watching television) consume as much or considerably more time. However, a very small subgroup of youth (between 3 and 6 percent) spends 20 or more hours/week participating; (3) There is quite consistent and strong evidence of a positive association between participating in organized activities and a variety of indicators of positive development: those youth who participate demonstrate healthier functioning on such indicators ranging from academic achievement, school completion, post secondary educational attainment, psychological adjustment, and lowered rates of smoking and drug use, to the quantity and quality of interactions with their parents. As the amount of participation in organized activities increases, the evidence suggests that the associated benefits of participation are observed either to accrue across the full range of activities or weekly hours of participation considered or to level off at relatively high amounts of participation; and (4) Concerning the well-being of youth with very high levels of involvement in organized activity participation (e.g., 20 or more hours/week), indicators of adjustment tended either to be more positive than, or similar to, youth who did not participate. Only a very few indicators of well-being have been shown to decline to a level significantly lower than youth who did not participate in organized activities. In sum, given the very limited empirical support for the over-scheduling hypothesis and the quite consistent support for the positive youth development perspective, we recommend that the recent efforts to expand opportunities for organized activity participation should stay the course. For the vast majority of young people, participation is associated with positive developmental outcomes. Of greater concern than the over-scheduling of youth in organized activities is the fact that many youth do not participate at all. The well-being of youth who do not participate in organized activities is reliably less positive compared to youth who do participate. (Contains 7 figures.)