Cognitive-behavioral therapy (CBT) encompasses various psychotherapeutic approaches that are rooted in the fundamental principle that a person’s thinking is the prime determinant of emotional and behavioral responses to life situations. The historical influences of CBT can be found in behavioral approaches such as John Watson’s classical conditioning and B. F. Skinner’s operant conditioning, with their focus on antecedents and reinforcers of behavior and an empirical approach to evaluating behavior, and Albert Bandura’s social learning theory and social cognitive theory, with a focus on observational or vicarious learning. Other influences that shaped the philosophical foundations of CBT include Greek and Roman Stoicism and the Eastern philosophies of Buddhism and Taoism, with their emphasis on reason, logic, and acceptance. Nonetheless, CBT as an organized system of psychotherapy, which originated during the 1950s and 1960s with the works of Albert Ellis’s rational-emotive behavior therapy (REBT) and Aaron Beck’s cognitive therapy (CT). Both of these models stress that cognitions, in the form of judgments, meanings, attributions, and assumptions tied to life events, are the primary factors that influence how individuals respond to environmental cues. The CBT emphasis on internal, private, conscious thought represents a departure from psychoanalytic theory, which emphasizes unconscious motivation of behavior, as well as from behaviorism, with its focus on external observable and measurable behaviors. Although there are differences among the various cognitive-behavioral approaches, there are fundamental similarities that include a focus on conscious thinking; the importance laid on information processing and the role that cognitions play in how we process information from our environment and respond to situations; and the assumption that, by changing irrational or maladaptive thoughts in a more rational, logical, realistic, and balanced perspective, people are capable of increasing healthy functioning. CBT is a present-oriented, relatively brief, structured, problem-focused, empirically driven form of psychotherapy. In CBT both the clinician and the client take an active approach in addressing the client’s problem. In a nutshell, the therapeutic work revolves around identifying maladaptive thinking; assessing the validity and functionality of such thoughts by evaluating available evidence for or against the thoughts; and formulating a more rational, logical, realistic, and balanced approach to interpreting one’s reality. In the late 20th and early 21st centuries various models of CBT have been applied to a wide range of mental health problems, substance abuse issues, and other disorders. More importantly, CBT has generated voluminous research studies, making it one of the most empirically based systems of psychotherapy.