This new book recognizes the reality that all principals are responsible for supervision, evaluation, and professional development of their teachers—tasks that are neither simple nor without conflict. The primary audience of this text is aspiring and practicing principals. We hope to help them understand both the theory and practice of supervision, evaluation, and professional development. Observing instruction, collection data for reflection, and having conversations about teaching however, are not sole provinces of principals. Master teachers, teacher leaders, and teacher colleagues can also benefit from the Supervisory sections of the book, especially the chapters on high-quality instruction, improving instruction, and the classroom data collecting tools.