Leech swimming is produced by the antiphasic contractions of dorsal and ventral longitudinal muscles that travel rearward along the animal and propel it forward. Research over the past three decades has focused on identifying the underlying neuronal circuit and mechanisms that produce and control this coordinated movement pattern. Investigations have also tested whether leech swimming is modifiable, both by experience and by neuromodulators. One outcome has been the identification of several functional classes of neurons associated with swimming. Systematic analysis of the interactions between these neurons had led to the elucidation of a neuronal circuit that adequately accounts for the generation of the swim motor program cord. The swim motor program appears to be produced by a chain of coupled segmental oscillators whose intrinsic properties and intersegmental connections ensure the coordinated expression of swimming along the nerve cord. In addition, neurons identified in the head ganglion comprise two parallel, but opposite-acting, systems that control the initiation of swimming in response to sensory input. Also, the pathway by which body wall stimulation initiates swimming shows a simple form of learning, that is habituation. Repeatedly stroking the leech body wall decreases both the probability of initiating swimming and the length of elicited swim episodes. Finally, the biogenic amine serotonin, which is found in the nerve cord, affects leech swimming in a number of ways. Serotonin's modulation of swimming is due, in part, to its effect of the membrane properties of swim-initiating interneurons and several swim motor neurons.