Neuronal circuits involved in left-right coordination are a fundamental feature of rhythmic locomotor movements. These circuits necessarily include commissural interneurons (CINs) that have axons crossing the midline of the spinal cord. The properties of CINs have been described in some detail in the spinal cords of a number of aquatic vertebrates including the Xenopus tadpole and the lamprey. However, their function in left-right coordination of limb movements in mammals is poorly understood. In this review we describe the present understanding of commissural pathways in the functioning of spinal cord central pattern generators (CPGs). The means by which reciprocal inhibition and integration of sensory information are maintained in swimming vertebrates is described, with similarities between the three basic populations of commissural interneurons highlighted. The subsequent section concentrates on recent evidence from mammalian limbed preparations and specifically the isolated spinal cord of the neonatal rat. Studies into the role of CPG elements during drug-induced locomotor-like activity have afforded a better understanding of the location of commissural pathways, such that it is now possible, using whole cell patch clamp, to record from anatomically defined CINs located in the rhythm-generating region of the lumbar segments. Initial results would suggest that the firing pattern of these neurons shows a greater diversity than that previously described in swimming central pattern generators. Spinal CINs play an important role in the generation of locomotor output. Increased knowledge as to their function in producing locomotion is likely to provide valuable insights into the spinal networks required for postural control and walking.