This book focuses on essential knowledge for teachers about rational numbers. It is organized around four big ideas, supported by multiple smaller, interconnected ideas--essential understandings. Taking teachers beyond a simple introduction to rational numbers, the book will broaden and deepen their mathematical understanding of one of the most challenging topics for students--and themselves. It will help teachers engage their students, anticipate their perplexities, avoid pitfalls, and dispel misconceptions. Teachers will also learn to develop appropriate tasks, techniques, and tools for assessing students' understanding of the topic. This book includes three chapters: (1) Rational Numbers: The Big Ideas and Essential Understandings; (2) Connections: Looking Back and Ahead in Learning; and (3) Challenges: Learning, Teaching, and Assessing. A foreword, a preface, an introduction, and a list of references are also included.