This book is designed to help mathematics teachers understand the rationale behind procedures regularly taught in the secondary mathematics classroom. The book begins by presenting a model of knowledge and understanding needed by teachers of mathematics. This model informs subsequent chapters in the book, addressing facets across five strands: Conceptually rich teacher explanations; Common student misconceptions; Guided discovery and active learning suggestions; Multiple mathematics representations to facilitate different learning styles; Authentic real life problems. This book is particularly suited for in-service and pre-service mathematics educators.