Behaviorism (or behaviourism), also called the learning perspective (where any physical action is a behavior), is a philosophy of psychology based on the proposition that all things that organisms do — including acting, thinking and feeling — can and should be regarded as behaviors. The school of psychology maintains that behaviors as such can be described scientifically without recourse either to internal physiological events or to hypothetical constructs such as the mind. Behaviorism comprises the position that all theories should have observational correlates but that there are no philosophical differences between publicly observable processes (such as actions) and privately observable processes (such as thinking and feeling).

From early psychology in the 19th century, the behaviorist school of thought ran concurrently and shared commonalities with the psychoanalytic and Gestalt movements in psychology into the 20th century; but also differed from the mental philosophy of the Gestalt psychologists in critical ways. Its main influences were Ivan Pavlov, who investigated classical conditioning, Edward Lee Thorndike, John B. Watson who rejected introspective methods and sought to restrict psychology to experimental methods, and B.F. Skinner who conducted research on operant conditioning.

In the second half of the twentieth century, behaviorism was largely eclipsed as a result of the cognitive revolution. Though these two schools of psychological thought may not agree theoretically, they have complemented each other in practical therapeutic applications. One notable legacy of behaviorist investigations is Cognitive-Behavioral Therapy, a popular treatment that uses cognitive models alongside behaviorist techniques such as 'systematic desensitization' and 'contingency management' that have demonstrable utility in helping people with certain pathologies, such as simple phobias, PTSD, and addiction.