The Gang Resistance Education and Training (G.R.E.A.T.) program is a gang- and delinquency-prevention program delivered by law enforcement officers within a school setting. Originally designed in 1991 by Phoenix-area law enforcement agencies to address local needs, the program quickly spread across the United States. In this article, the authors describe the evolution of the program and its responsiveness to two independent national evaluations funded by the U.S. National Institute of Justice. The first evaluation revealed little program effect and contributed to a critical review and substantial revision of the G.R.E.A.T. "core" or middle-school curriculum. Preliminary findings from the ongoing second evaluation give an initial indication of the extent to which these changes have resulted in the achievement of G.R.E.A.T. program goals of helping youths to (a) avoid gang membership, violence, and criminal activity; and (b) develop a positive relationship with law enforcement. (Published Abstract)