Behaviorism is used in somewhat different senses in psychology and philosophy. In psychology, behaviorism was a twentieth-century movement that maintained that the study of behavior is the best or even the only way to study mental phenomena scientifically. It is opposed to the introspective methods for the study of the mind emphasized in much psychology of the nineteenth century. This is methodological behaviorism. A methodological behaviorist might even believe in an immaterial mind (see dualism), but maintain nevertheless that there was no scientific way to study the immaterial mind except through its effects on observable, bodily behavior.

In philosophy, behaviorism opposes dualism; the term means some form of the view that the mind is nothing above and beyond behavior.

Logical behaviorists maintain that talk about the mind can be reduced without remainder to talk about behavior. Criteriologial behaviorists maintain that mental terms may not be completely reducible to behavioral term, but they can only be given meaning through ties to behavioral criteria.

Behaviorism is closely related to functionalism.