Building on the background in optics and waves covered in the last chapter, this chapter gives a brief review of quantum mechanics, focusing on those aspects that will be important in later parts in this book. It is assumed that the reader has at least some prior familiarity with quantum mechanics, so the goal here is partly to refresh the reader’s memory and fill in any gaps. Along the way, a few new ideas such as fidelity and decoherence, which are not normally taught in introductory quantum courses, will be introduced for later use. Along with the previous chapter, this one establishes a foundation for the remainder of the book.