Operant conditioning, also called instrumental conditioning, is a learning process in which voluntary behaviors are modified by association with the addition (or removal) of reward or aversive stimuli. The frequency or duration of the behavior may increase through reinforcement or decrease through punishment or extinction.

Operant conditioning originated with Edward Thorndike, whose law of effect theorised that behaviors arise as a result of consequences as satisfying or discomforting. In the 20th century, operant conditioning was studied by behavioral psychologists, who believed that much of mind and behaviour is explained through environmental conditioning. Reinforcements are environmental stimuli that increase behaviors, whereas punishments are stimuli that decrease behaviors. Both kinds of stimuli can be further categorised into positive and negative stimuli, which respectively involve the addition or removal of environmental stimuli.

Operant conditioning differs from classical conditioning in both mechanism and outcome. While classical conditioning pairs stimuli to produce involuntary, reflexive behaviors (like salivating at food), operant conditioning shapes voluntary behaviors through their consequences. Actions followed by rewards tend to be repeated, while those followed by negative outcomes diminish.

The study of animal learning in the 20th century was dominated by the analysis of these two sorts of learning, and they are still at the core of behavior analysis. They have also been applied to the study of social psychology, helping to clarify certain phenomena such as the false consensus effect.

Operant conditioning, sometimes called instrumental learning, was first extensively studied by Edward L. Thorndike (1874–1949), who observed the behavior of cats trying to escape from home-made puzzle boxes. A cat could escape from the box by a simple response such as pulling a cord or pushing a pole, but when first constrained, the cats took a long time to get out. With repeated trials ineffective responses occurred less frequently and successful responses occurred more frequently, so the cats escaped more and more quickly. Thorndike generalized this finding in his law of effect, which states that behaviors followed by satisfying consequences tend to be repeated and those that produce unpleasant consequences are less likely to be repeated. In short, some consequences strengthen behavior and some consequences weaken behavior. By plotting escape time against trial number Thorndike produced the first known animal learning curves through this procedure.

Humans appear to learn many simple behaviors through the sort of process studied by Thorndike, now called operant conditioning. That is, responses are retained when they lead to a successful outcome and discarded when they do not, or when they produce aversive effects. This usually happens without being planned by any "teacher", but operant conditioning has been used by parents in teaching their children for thousands of years.

B.F. Skinner (1904–1990) is referred to as the Father of operant conditioning, and his work is frequently cited in connection with this topic. His 1938 book "The Behavior of Organisms: An Experimental Analysis", initiated his lifelong study of operant conditioning and its application to human and animal behavior. Following the ideas of Ernst Mach, Skinner rejected Thorndike's reference to unobservable mental states such as satisfaction, building his analysis on observable behavior and its equally observable consequences.

Skinner believed that classical conditioning was too simplistic to be used to describe something as complex as human behavior. Operant conditioning, in his opinion, better described human behavior as it examined causes and effects of intentional behavior.