Locomotor behaviors, such as flying, swimming or walking, are fundamental motor acts that give animals and humans the ability to move. Localized neuronal networks called central pattern generators, or CPGs, generate much of the timing and pattern of these complex movements. CPGs controlling walking are located in the spinal cord and are found in all vertebrates, including humans. New knowledge on the mammalian walking CPG is advancing rapidly (1). In this talk I will review findings that have identified CPG neurons involved in the left-right coordination. These neurons play an important role in coordinating flexor and extensor activity between the two sides of the body. I will also review molecular and genetic strategies which, when combined with a classical electrophysiological approach to network analysis, hold promise to give unprecedented insight into the network structure. Use of these methods that include genetically modified mice with selctive labeling of neurons, knockout and/or acute silencing of spinal neurons have identified a group of ipsilateral projecting inhibitory neurons that play an important role for the control of speed (2), as well as groups of excitatory neurons that might be directly involved in rhythm generation (3–4). Understanding the locomotor CPG in mammals is also an important step in improving clinical neuro-rehabilitation of patients with spinal cord injury. 1. Kiehn, O. (2006) Ann Rev Neurosci, 29:279–306. 2. Gosgnach S, Lanuza GM, Butt SJ, Saueressig H, Zhang Y, Velasquez T, Riethmacher, D, Callaway EM, Kiehn O, Goulding M (2006). Nature 440:215–9. 3. Kullander K. , Butt S.J.B., Lebret J., Lundfald L., Restrepo E., Klein R., and Kiehn, O. (2003). Science 299: 1889–1892. 4. Butt SJ, Lundfald L, Kiehn O. PNAS 102:14098– 103.