Cognitive behavioral therapy (CBT) is the predominant form of psychological therapy, with a substantial evidence base of randomized controlled trials across a wide range of mental health disorders. Overturning the earlier assumption that psychotherapeutic interventions were unsuitable for people with intellectual and developmental disabilities (IDD), CBT is now in widespread use by clinical psychologists working with people with IDD, particularly in the UK. The first section of this chapter reviews briefly the basic principles of CBT, and considers the need to adapt CBT for people with IDD, which arises from the fact that, in addition to the cognitive distortions that are the major focus of CBT interventions, it is also necessary, when working with people with IDD, to take account of cognitive deficits in the areas of emotional literacy, memory, executive functioning, and self-direction. We consider the kinds of adaptations that may typically be needed in relation to each of the specific components of a CBT intervention. We next review the evidence for the effectiveness of CBT interventions for people with IDD. This includes a large number of controlled trials of interventions for problem anger, a much smaller literature on interventions for depression, and a relatively large body of case-study material in other conditions. Though, for the most part, methodologically weak, the evidence suggests that the effectiveness of CBT for people with IDD is comparable to that seen in the general population. Finally, we present some practice recommendations in relation to screening, formulation, overcoming dependency and stigma, promoting motivation and engagement, and setting an appropriate pace of treatment.