As you have probably already guessed by the title, this book is not a page-turner; there are no life-changing secrets buried here. Rather, it is meant to serve as a resource for both seasoned professionals in and newcomers to the healthcare industry, a resource that provides concise but thorough descriptions of select clinical classification systems and select healthcare databases. We have followed a very simple format. Each entry on a named system or database presents a general description of its current state, followed by a timeline of key points in its evolution. Finally, a summary section describes present structure and terminology. This arrangement will, we hope, help the reader understand the antecedents and evolution of these systems and databases, and thereby help guide the choice of exactly the right such resources for a particular project. Which systems and databases align best with the questions you hope to answer, the information you hope to impart? The information presented here should make those decisions and selections faster, easier, smoother, and more confident.