The notion of the command cell has been highly influential in invertebrate neurobiology, and related notions have been increasingly used in research on the vertebrate nervous system. The term “command neuron” implies that the neuron has some critical function in the generation of a normally occurring behavior. Nevertheless, most authors either explicitly or implicitly use a strictly operational definition, independent of considerations of normal behavioral function. That is, command neurons are often defined as neurons which, when stimulated by the experimenter, evoke some behavioral response. Even when utilizing such an operational definition, investigators frequently differ on what they consider to be the exact characteristics that a neuron must have (or not have) to be considered a command cell. A few authors appear to treat command neurons in relation to normal function, but a precise behaviorally relevant definition has not been specified. Because of the ambiguity in the definition of command neurons, the term can refer to a wide variety of neurons which may play divergent behavioral roles. In some ways the attempt to label a cell as a command neuron may interfere with the process of discovering the complex causal determinants of behavior. Nevertheless, the notion that individual cells are responsible for certain behaviors is highly appealing, and an attempt to define the command neuron rigorously could be worthwhile. We suggest that a command neuron be defined as a neuron that is both necessary and sufficient for the initiation of a given behavior. These criteria can by tested by: (1) establishing the response pattern of the putative command neuron during presentation of a given stimulus and execution of a well specified behavior; (2) removing the neuron and showing that the response is no longer elicited by the stimulus (necessary condition); and (3) firing the neuron in its normal pattern and showing that the complete behavioral response occurs (sufficient condition). In some cases, groups of neurons, when treated as a whole, may satisfy the necessity and sufficiency criteria for a given behavior, even though individual neurons of the group fail to meet the criteria. We suggest that such a group be termed a “command system” for the behavior in question. Individual neurons in the command system can be termed “command elements” if, when fired in their normally occurring pattern, they elicit a part of the behavior, or “modulatory elements” if they do not in isolation elicit any response, but alter the behavior produced by other elements in the command system.