Since the experiments of Nicholls and Baylor, the initial characterization of identified neurons has provided significant insight into the circuitry transforming mechanosensory input into the motor output of swimming. From physiological characterization of only a small percentage of cells within the leech CNS, we have gained important information about how the decision to swim is processed and how the rhythmic motor pattern is generated. While many of the synaptic connections in the swim-generating circuit have been identified, the elucidation of the biophysical and biochemical mechanisms underlying these connections has only recently begun. The observation that constant input can result in variable motor output suggests that, in addition to describing a cell's identity in terms of structure and function, factors such as behavioral context and the "internal state" of the nervous system must also be considered. As circuits controlling other behaviors become known, one can examine the interactions between these networks to understand issues of behavioral choice at the level of identified neurons. The leech CNS has expanded our understanding of how the nervous system produces behavior and continues to serve as an excellent model in this endeavor.