Chapter 1 is an introduction of the book. This chapter provides a brief explanation on what is the motivation for the author to write this book and why this book is focused on quaternion-based design in spacecraft modeling, attitude determination, and control. The main body of this introduction is a description that summarizes the content of the rest of the chapters. The first 10 chapters are the fundamental materials that should be understood by all the readers who are interested in spacecraft attitude determination and controls. Chapters 11–15 are relatively independent of each other. These topics are important, the readers may choose not to read some of these chapters in the beginning but later. The purpose of this summary is to let the readers make a decision on whether they want to skip reading some material in the first go and read it later. Some notations used throughout the book have been provided at the end of this chapter.