Research Summary: This manuscript presents results from the National Evaluation of the Gang Resistance Education and Training (G.R.E.A.T.) program, a school-based prevention program targeting middle-school students. A longitudinal quasi-experimental research design was conducted from 1995 through 1999. Beneficial program effects emerged gradually over time so that there was, on average, more pro-social change in the attitudes of G.R.E.A.T. students than the non-G.R.E.A.T. students four years following program exposure. Policy Implications: Two specific policy recommendations stem from this research. First, law enforcement officers can be effective providers of school-based prevention programs. Second, to better assess program effectiveness, evaluations should include design features that allow for assessment of long-term or delayed program effects.