Conditioning is a way of establishing new behaviors by providing either a stimulus or a reward for the desired behavior. It has widespread applications in psychology. There is also growing evidence that conditioning might be useful to boost the body's immunity against disease, or to suppress the immune system's tendency to reject transplanted organs.

There are two main types of conditioning: classical and operant.

The best-known examples of classical conditioning are the experiments conducted in the late 1800s and early 1900s by the Russian physiologist Ivan Pavlov. Initially, Pavlov was interested in the functioning of the digestive system (he received the 1904 Nobel Prize in Medicine and Physiology for that work). In experiments with dogs, he developed an experimental apparatus for measuring their saliva formation as they ate, using tubes that redirected secretions from the animals' salivary ducts. While conducting these experiments, Pavlov noticed that some dogs started salivating...