Participation in various structured out-of-school time (OST) activities benefits youth socially, emotionally, and academically and may have the most positive effects for youth who are most at risk. Little research, however, has explored the questions of who participates in OST activities and why. The Harvard Family Research Project examined the factors associated with whether youth--particularly disadvantaged youth--participate in organized out-of-school time (OST) programs and activities. This Study of Predictors of Participation in Out-of-School Time Activities used national data sets that provided valuable information about the child, family, school, and neighborhood predictors of OST participation. This brief highlights key findings for OST practitioners and policymakers as they work to address issues of access and equity, document service gaps, and target resources accordingly. Specifically, the brief addresses what predictive factors, including family income and ethnicity, predict participation in OST activities, and whether OST participation among disadvantaged youth has increased in recent years.