Keller and Shoenfeld’s Principles of Psychology, published in 1950, was written as an introductory text to be used in the two-semester Psychology 1-2 course at Columbia University. It is a systematic approach in that a small number of functional relations described in B. F. Skinner’s The Behavior of Organisms are introduced and then used throughout to interpret the topics presented in a typical introductory psychology course. K & S was widely influential in familiarizing psychologists and others with the nature and general relevance of Skinner’s approach. It is an outstanding example of clear and interesting technical writing, and its style and topic arrangement have been the basis for a number of subsequent texts. Although old by textbook standards, it is still one of the easiest ways to acquire an accurate repertoire in the science of behavior.