Schools throughout the country are now encouraged to implement school-wide positive behavior support (PBS) procedures as a way to improve their behavioral climate, safety, and social culture. Research is needed to determine (a) the extent to which schools already use school-wide PBS, (b) if training and technical assistance efforts result in change in the use of school-wide PBS procedures, and (c) if use of these procedures is related to valued change in safety, social culture, and behavior within schools. To address these questions, researchers need a metric for assessing implementation of school-wide PBS practices. The "School-Wide Evaluation Tool" (SET; Sugai, Lewis-Palmer, Todd, & Horner, 2001) was created to provide a rigorous measure of primary prevention practices within school-wide behavior support. In this article, the authors describe the SET and document its psychometric characteristics. The results of their study suggest that the SET is a valid, reliable measure that can be used to assess the impact of school-wide training and technical assistance efforts. The SET should also be useful in formal analyses of the relationship between use of school-wide PBS and changes in social and academic outcomes. (Contains 7 tables and 1 figure.)