For over a century, psychologists have been conducting experimental studies, largely with nonhuman animals, in order to establish general principles of learning and behavior. In everyday usage, the term “learning” refers to the acquisition of new information or new skills. But psychologists often have a wider interpretation, taking learning to refer to almost any fairly permanent change in an animal’s behavior, or potential for behavior, that results from experience. Learning is the process whereby an animal interacts with its environment and becomes changed by the experience so that its further behavior is modified. “Learning theory” refers to the attempt to devise an account of the mechanisms responsible for this process of individual adaptation. The data it deals with are largely, but not exclusively, derived from experimental studies of the behavior of nonhuman animals, tested in controlled conditions. The last century, starting with the pioneering work of Pavlov (on classical conditioning) and of Thorndike (on instrumental learning), saw the development of a range of rival learning theories that are still of present-day concern. The dominant theoretical perspective may be termed associative (or alternatively, connectionist); it explains learning in terms of the formation of hypothetical links (presumed to be in the central nervous system) between entities representing external events or responsible for the emission of behavior. This central notion has been extended by theorists who emphasize the role of motivational factors that activate such links, or who postulate a wider range of psychological processes (conveniently labeled cognitive: e.g., attention, rehearsal), with association formation given a lesser role. Finally, some have rejected not just the notion of association but all postulation of central psychological processes. This atheoretical “theory,” referred to as radical behaviorism or the experimental analysis of behavior, is primarily associated with the name of Skinner and focuses on instrumental learning (also called operant conditioning). Although learning theories have been derived principally from work on laboratory animals, they are intended to be applicable more generally. Their findings and concepts are relevant to understanding the psychology of domestic and wild animals in their natural habitats and thus contribute to the growing field of research known as animal cognition. They also apply to the behavior of our own species. Experimental studies of human subjects, based on paradigms developed with animals, can thus be used to test and develop our theories. More important, the methods and findings of learning theory have direct application to issues of human concern. Proponents of the experimental analysis of behavior have promoted the use of training procedures involving positive reinforcement for addressing a number of behavior disorders and as the basis for education more generally. Theoretical concepts developed in the associative tradition have been used to illuminate the source of (and possible treatments for) clinical problems such as anxiety disorders and drug addiction.