This volume shows how evaluators can play a vital role in shaping publicpolicy by testing the validity of widely held beliefs. The contributorsprovide concrete examples of evaluation results that unexpectedly forcedpolicymakers to modify or completely revamp programs in different humanservice fields. Chapters examine how evaluation results lead to a dramaticrestructuring of programs for reducing recidivism for domestic violence andpreventing perinatal transmission of HIV , and offer evaluation-basedevidence that privatization actually results in greater, rather than lesser,demand for public services. They also focus on the influence of evaluationon educational policies, illustrating how better cost-effectiveness analysiscan support change efforts, how narrative evaluations can actuallystrengthen standardized test results, and how evaluation can help todetermine the most successful professional development programs forteachers. This is the 90th issue of the Jossey-Bass series New Directions for Evaluation.