This chapter reviews the four main psychological models and theories of generalized anxiety disorder (GAD). These are cognitive avoidance theory, metacognitive model, intolerance of uncertainty theory, and the emotional dysregulation model. The empirical support for each is critically appraised. Although each theory offers a cognitive behavioral analysis, they differ in their emphasis on the psychological processes involved in the maintenance of pathological worry. As a result they give rise to four distinct treatments. The clinical implications and the treatment efficacy of the four approaches are briefly reviewed. The chapter concludes with a comparative analysis of how well each theory accounts for the cardinal feature of GAD, namely, excessive and uncontrollable worry.