Book excerpt: This book deals with the most important individual, professional development, program, and system factors that influence the type and amount of teacher change. This study was conducted primarily to help professional development decision-makers plan and deliver effective professional development, and to understand the factors that influence how teachers change as a result of professional development. How do practitioners change as a result of participating in one of three different models of professional development, and what are the most important factors that influence (support or hinder) this change?