The vertebrate spinal cord contains a central pattern generator or CPG that can produce the rhythmic movements of locomotion in the absence of peripheral and descending inputs. CPGs serve as relatively simple model systems for understanding how neuronal assemblies produce complex functions. While the organization of the constituent interneurons in the CPG controlling swimming in non-limbed animals is known in great detail, less is known about the cellular organization of CPGs controlling walking in limbed mammals. In this talk I will review findings that have identified CPG neurons involved in the left-right alternation during locomotion. These neurons play and important role in coordinating flexor and extensor activity between the two sides of the body. Use of genetically modified mice including selective labeling, knockout and/or acute silencing of spinal neurons have also identified a group of ipsilateral projecting inhibitory neurons that plays an important role for control of speed as well as groups of excitatory neurons that might be dircetly involved in rhythm generation. Together these data comprise characterization of essential elements of the mammalian walking CPG Butt, S. J. and Kiehn, O. (2003). Neuron, 38(6), 953–963; Kullander K. , Butt S.J.B., Lebret J., Lundfald L., Restrepo E., Klein R., and Kiehn, O. (2003). Science 299, 1889–1892. Butt SJ, Lundfald L, Kiehn O. (2005). PNAS, 102(39):14098–103.