Behavior analysis today has moved far beyond the simple response-reward conditioning of the past. While still embracing these concepts, modern behavior analysis recognizes that traditional behavioral processes can give rise to other behaviors (e.g., rule governance, relational framing) that can actually alter the way these processes function-a sort of recursive, behavior-modifying-behavior. Traditional behavioral conceptualizations of various behavioral disorders failed to incorporate these function-altering behavioral processes, and as a result, non-behavior analytic models of these disorders were developed to account for the oversights. Behavior analytic theory came to be regarded as too narrow to account for the complexities involved in human pathology. But recent research on the behavior analysis of human language and cognition (e.g., Hayes, Barnes-Holmes, and Roche, 2001) have enabled behavior analysis to regain its theoretical foothold in the description of behavior disorders. This book provides a working and testable theory of common behavior disorders from a modern behavioral perspective. It covers concepts such as rule-governance, experiential avoidance, and relational framing in addition to traditional behavioral concepts such as reinforcement, punishment, establishing operations, and stimulus control. Most of the theories presented in the book reach beyond the current body of behavior analytic research because most behavior disorders have not been examined through a modern behavior analytic perspective. But the authors describe their behavior analytic model and search for the nonbehavioral research that is consistent with their theory. Throughout, the book presents a logical, plausible, and testable theory that is consistent with modern behavior analytic thinking.