The mechanisms of behavior have traditionally interested psychologists, but in recent years the analysis of behavior has also attracted a number of neuro­ biologists. One reason for the neurobiologists’ interest is that the study of behavior has advanced dramatically during the last decades. Another reason is that recent progress in the analysis of nerve cells and of synaptic transmission has encouraged neurobiologists to look beyond cellular function, which they are beginning to understand, to the analysis of systems of cells. Among the most interesting neural systems for study are those that control complete behavioral acts. Here one can examine how an interconnected group of cells mediates behavior and how simple forms of learning, such as habituation and sensitiza­ tion, produce alterations in cellular function.