Classical conditioning focuses on a behavior that invariably follows a particular event, whereas operant (or instrumental) conditioning concerns the learning of behavior that operates on the environment: The person or animal behaves in a particular way to gain something desired or avoid something unpleasant. This behavior is initially emitted rather than elicited–you wave your hand to flag down a taxi, dogs beg at the dinner table to get food.

Psychologist Edward Lee Thorndike was the first researcher to study operant behavior systematically. He used a "puzzle box" to determine how cats learn.

Thorndike's work still stands as a landmark in our understanding of the effects of both reinforcers and punishers. In operant conditioning, reinforcement (such as food) is used to increase the probability that a particular response will occur in the future. To decrease the probability that a particular response will recur, punishers (such as scolding) are used. Thorndike proposed the law of effect, which states that behavior that is consistently rewarded will become "stamped in" as learned behavior and behavior that is consistently punished will be "stamped out."

There are several kinds of reinforcers; all of them strengthen behavior just as steel rods reinforce or strengthen concrete.

The presence of positive reinforcers (such as food) adds to or increases the likelihood that a behavior will recur. Negative reinforcers (such as terminating electric shocks) also increase the likelihood that a behavior will recur, but they do so by reducing or eliminating something unpleasant from the environment.

Although all reinforcers (both positive and negative) increase the likelihood that a behavior will occur again, punishment is any event whose presence decreases the likelihood that ongoing behavior will recur. Reinforcement always strengthens behavior; punishment weakens it. Avoidance training involves learning a desirable behavior that prevents an unpleasant condition, such as punishment, from occurring.

Studies have revealed that in operant conditioning the behaviors that are easiest to condition are those that animals typically would perform in the training situation. These behaviors vary from species to species, and put significant constraints on both classical and operant conditioning.

When something we do is followed closely by a reinforcer, we tend to repeat that behavior, even if it was not actually responsible for producing the reinforcement. Such behaviors are called superstitious. Nonhumans as well as humans exhibit superstitious behaviors.

The failure to avoid or escape from an unpleasant or aversive stimulus that occurs as a result of previous exposure to unavoidable painful stimuli is referred to as learned helplessness. Learned helplessness, which has been demonstrated in both animals and humans, is associated with many of the symptoms characteristic of depression.