B. F. Skinner titled this book, Contingencies of Reinforcement, after the heart of his science of behavior. Contingencies relate classes of actions to postcedent events and to the contexts in which those action-postcedent relations occur. The basic processes seem straightforward, but many people do not know or understand the underlying theory. Skinner believed that ‘a theory is essential to the scientific understanding of behavior as a subject matter”. This book presents some of Skinner’s most sophisticated statements about theoretical issues. To his original articles, he added notes to clarify and expand subtle points. The book thus provides an overview of Skinner’s thinking about theory and the philosophy underpinning the science he began.