Cognitive behavior therapy (CBT) has come to be a widely practiced psychotherapy throughout the world. The present article reviews theory, history, and evidence for CBT. It is meant as an effort to summarize the forms and scope of CBT to date for the uninitiated. Elements of CBT such as cognitive therapy, behavior therapy, and so-called “Third wave” CBT, such as dialectical behavior therapy (DBT) and acceptance and commitment therapy (ACT) are covered. The evidence for the efficacy of CBT for various disorders is reviewed, including depression, anxiety disorders, personality disorders, eating disorders, substance abuse, schizophrenia, chronic pain, insomnia, and child/adolescent disorders. The relative efficacy of medication and CBT, or their combination, is also briefly considered. Future directions for research and treatment development are proposed.