Research Summary: Using multi-level modeling techniques, this study explores characteristics of 35 after-school programs (ASPs) that criminological research and theory predict should be related to problem behavior outcomes. Controlling for individual-level predictors of problem behavior and for the composition of the participating ASPs, several ASP characteristics were found to be related, as predicted, to victimization, substance use, and delinquent behavior. Policy Implications: This study extended previous findings that providing structured programming and small program size are important for reducing problem behavior through ASPs. Our study also found that two characteristics of the program staff are related to reductions in problem behavior: More highly educated staff and a higher percentage male staff were related to reductions in levels of both delinquent behavior and victimization. The study concludes that program structure, staffing, and size are important in producing more positive behavioral outcomes.