In a typical experiment on classical or Pavlovian conditioning, a subject is repeatedly exposed to a sequence of two events. The initial event in the sequence is a relatively neutral stimulus that may not support overt responses (an initial ori enting reaction to the stimulus habituates after several presentations). This is referred to as the conditioned stimulus (CS). The other element in the sequence is a relatively strong stimulus ca pable of eliciting one or several responses (e.g., movements or autonomic reactions). This is referred to as the unconditioned stimulus (US) or reinforce?.; and the response it elicits is named the unconditioned response (UR). Such CS-US pairings are separated by an intertrial interval (ITI) and are presented several times within a particular session. Eventually the CS comes to control the occurrence of some response as a result of such CS-US pairings, called the condi tioned response (CR). This basic procedure, called “delay conditioning,” was developed and studied in great detail by Pavlov (1927). As will be seen shortly, there are many departures from delay procedures and important qualifications of the outcome just described.