C lassical conditioning is a fundamental form of learning and memory by which organisms learn to associate which stimuli in the environment lead to reinforcing and punishing outcomes. This form of learning is critical to survival, as it helps organisms identify likely sources of food in the environment and situations and places where a predator is likely to be encountered. Although theoretical accounts of classical conditioning were initially described in terms of reexive chaining of simple stimulus-response associations, it is now known that many levels of learning occur when organisms evaluate the conditions that lead to biologically salient events (Rescorla, 1988). A century ago, Ivan Pavlov (1927) discovered the basic tenets of classical conditioning through his famous physiologic experiments on salivation of dogs in response to the ringing of bells that predict food delivery. Although Pavlov described a form of conditioned learning for items that an organism wants (known as appetitive learning), recent research in neuroscience has focused more on learning the relationships among stimuli that predict delivery of noxious stimuli, resulting in the acquisition of fear responses that occur through conditioning.