Ever since the discovery of the chemical nature of synaptic transmission, the role of neurotransmitters in the formation and retrieval of memories has been the subject of intense scientific investigation. As the number of both neurotransmitters and forms of memories has been steadily increasing over the years, the task of uncovering general principles describing the involvement of neurotransmitter systems in memory has become extremely difficult. Furthermore, the lack of understanding of the molecular and cellular mechanisms of learning and memory has limited the experimental approaches to two general strategies: (1) an interventional strategy using pharmacological tools or lesion/stimulation of specific neurotransmitter systems; and (2) a correlational strategy using "naturally" occurring conditions (neurological diseases, aging) affecting specific neurotransmitter systems, or genetically engineered mutant mice. Based on these studies, a number of neurotransmitters and neuronal pathways using these neurotransmitters have consistently demonstrated an important role...