Intrinsic spinal networks, known as central pattern generators (CPGs), control the timing and pattern of the muscle activity underlying locomotion in mammals. This review discusses new advances in understanding the mammalian CPGs with a focus on experiments that address the overall network structure as well as the identification of CPG neurons. I address the identification of excitatory CPG neurons and their role in rhythm generation, the organization of flexor-extensor networks, and the diverse role of commissural interneurons in coordinating left-right movements. Molecular and genetic approaches that have the potential to elucidate the function of populations of CPG interneurons are also discussed.