This volume shows how evaluators can play a vital role in shaping public policy by testing the validity of widely held beliefs. The contributors provide concrete examples of evaluation results that unexpectedly forced policymakers to modify or completely revamp programs in different human service fields. Chapters examine how evaluation results lead to a dramatic restructuring of programs for reducing recidivism for domestic violence and preventing perinatal transmission of HIV, and offer evaluation-based evidence that privatization actually results in greater, rather than lesser, demand for public services. They also focus on the influence of evaluation on educational policies, illustrating how better cost-effectiveness analysis can support change efforts, how narrative evaluations can actually strengthen standardized test results, and how evaluation can help to determine the most successful professional development programs for teachers. This is the 90th issue of the Jossey-Bass series "New Directions for Evaluation."