Tidying up messy software is a must. And that means breaking up the code to make it more readable -- "tidying" the code to make it more understandable. In this practical guide, author Kent Beck, creator of Extreme Programming and pioneer of software patterns, suggests when and where you might apply tidyings in your code to improve it while keeping the overall structure of the system in mind. Instead of trying to master tidying all at once, this book lets readers try out a few examples that make sense for their problem. If they have a big function containing many lines of code, they'll learn how to logically divide it into smaller chunks. Along the way, developers will learn the theory behind software design: coupling, cohesion, discounted cash flows, and optionality. This book helps you: Understand the basic theory of how software design works, and learn about the forces that act on it Explore the difference between changes to a system's behavior and changes to its structure Improve your programming experience by sometimes tidying first and sometimes tidying after Learn how to make large changes in small, safe steps Prepare to design as a human activity with diverging incentives.