This talk will examine two aspects of the organization of spinal circuits devoted to the control of limb movement – the logic of circuit assembly, and the link between circuit wiring and motor behavior. The application of molecular genetic methods to problems of neuronal diversity and connectivity has begun to emphasize the importance of neuronal settling position as an adjunct to cell recognition in driving the assembly of motor microcircuits. Through this analysis it has become evident that the detailed wiring of local motor circuits differs systematically, as a function of limb muscle position and the biomechanical demands of individual joints. Molecular appreciation of the vast diversity of spinal motor neuron and interneuron subtypes has also permitted more precise ways of perturbing, genetically, the function of individual spinal neuronal subsets and examining the consequences of such manipulations for motor behavior. Examples of the way in which this general approach can be applied to questions of local inhibitory control, and the internal representation of motor output, will be discussed.