This article examines links between different measures of after-school time activity participation (5 specific activities and breadth) on youth's developmental outcomes (anxiety/depression, delinquency, and substance use) over 6 years and whether these links are moderated by neighborhood-level variables. The sample (N = 1,315) of 9- and 12-year-old youth was drawn from the Project on Human Development in Chicago Neighborhoods (PHDCN), a multilevel, longitudinal study of youth from 80 Chicago neighborhoods. Findings revealed that different types of activities and patterns of participation over time were associated with outcomes for youth and that, to some extent, these outcomes varied with neighborhood characteristics. In brief, sports participation was associated with fewer anxious/depressed symptoms, higher average delinquency scores, and increased substance use-both average scores and growth over time. Participation in the arts and student government were negatively associated with average substance use and attenuated increases in usage over time. Participation in community-based clubs was positively associated with youth's anxiety/depression in violent neighborhoods only, whereas church groups were protective against substance use in nonviolent neighborhoods. The direction of the influence of breadth of participation was nonlinear for delinquency such that delinquency scores were highest among youth who engaged in an average number of activities.