In 1953 Olds and Milner 9o discovered that animals would repeat acts that were followed by delivery of electrical stimulation of the brain. This finding had important implications for theoretical issues at the time, and it has led to one of the most prominent experimental lines of study of brain-behavior relationships. The theoretically important issue of the time was whether reward or reinforcement was a phenomenon of learning which emerged from general properties common to all nerve cells, or whether there were specialized neurons in the brain which were essential for learning and reinforcement 89 and which played a unique role in these phenomena without necessarily being part of the memory engram. The fact that brain stimulation was rewarding with some electrode placements but not others was interpreted as meaning that there were such specialized reward systems in the brain. Later studies showed that brain stimulation reward followed the laws of reinforcement as derived from studies of food and water reward 147, and subsequent thinking about brain mechanisms of reinforcement came to involve notions of'reward centers','reward neurons','reward systems','reward synapses' and'reward transmitters'. The study of brain mechanisms of reinforcement has largely been dominated by studies involving intracranial electrical stimulation, and the'reward systems' as suggested by brain stimulation studies have been assumed to represent systems which are normally activated by more natural reinforcers such as food for hungry animals. In the past decade and a half, thinking about reward systems in the brain has been greatly influenced by one form or another of the catecholamine (CA) hypothesis. In its simplest form, this hypothesis suggests that some'reward neurons' contain a catecholamine as their neurotransmitter. More carefully stated, the position is that there exist one or more central neural systems which are specialized for, and which play a critical role in, reward phenomena, and that at least one critical link in the system or