Behaviorism is a branch of psychology which focuses on the study of observable behavior, with the accompanying belief that all human activities, from feeling an emotion to performing a physical task, are forms of behavior. Behaviorists are interested in what they can observe, quantify, and manipulate, looking at the impacts of environmental stimuli on the organisms they study. These researchers work with a wide variety of animals, including humans, to learn more about why they do what they do.

The father of behaviorism is John B. Watson, who coined the term in 1913, claiming that he wanted to turn the focus of psychology to the study of behavior, rather than nebulous exploration of the mind. Many other scientists and researchers took up the discipline and expanded it, with a notable behaviorist being B. F. Skinner, a researcher who worked in the mid to late 20th century.

According to behaviorists, everything is a form of behavior which occurs in response to stimuli in the environment. This includes things like critical thinking and problem solving, completion of physical tasks, and the experience of emotions. While behaviorists acknowledge that cognitive processes are occurring and can be involved in behavior, they stress that these processes occur in response to stimuli, making the outcome of such processes a form of behavior.

Behaviorism focuses heavily on the use of conditioning, the idea that stimuli can be used to teach organisms to repeat or avoid behavior. In fact, conditioning can be used to manipulate the behavior of an organism to create a desired outcome. For example, people use conditioning in animal training to teach animals to do things like canter on a command from a rider, sit when signaled to do so, or attack when ordered to do so by a handler.

Under the system of behaviorism, numerous things can be quantified, manipulated, and explored to learn more about the behavior of organisms from ants to elephants. Some other fields have integrated some concepts from behaviorism, such as the idea of operant conditioning to promote a desired behavior, and some behaviorists pursue more or less radical forms of behaviorism in their work.

Some opposition has been lodged against behaviorism. For example, some theologians argue that this discipline would seem to reject the existence of a God or higher power by eliminating free will and treating humans essentially like machines. Other people within the field of psychology have also argued that behaviorism incompletely explains maladaptive behavior or problems which appear to be of a psychological, rather than behavioral, origin.