This book introduces the many benefits of mathematics professional development for middle school teachers. It explores key features of effective professional development, research-based models, that can be used to design professional development and an analysis of professional development in the context of adult-learning theories. The importance of professional learning communities as an avenue for educators to grow in their knowledge and skills is also discussed. A qualitative research study that elucidates the many benefits of mathematics professional development on teachers' knowledge, skills, and dispositions to teach middle school students is explored. The results of the study and its implications to teaching practice are discussed.