In the walking system of the cat, feedback from muscle proprioceptors establishes the timing of major phase transitions in the motor pattern, contributes to the production of burst activity, generates some features of the motor pattern, and is required for the adaptive modification of the motor pattern in response to alterations in leg mechanics. How proprioceptive signals are integrated into central neuronal networks has not been fully established, largely due to the absence of detailed information on the functional characteristics of central networks in the presence of phasic afferent signals. Nevertheless, it appears likely that afferent signals reorganize the functioning of central networks, and the concept that the generation of the motor pattern can be explained by afferent modulation of a hard-wired central pattern generator may be too simplistic.