How does the assembly of neuronal circuits contribute to the emergence of function controlling dedicated animal behaviors? Finding answers to this question requires a deep understanding of the connectivity map of neuronal circuits controlling a behavior as well as the mechanisms involved in the generation of these specific circuit maps. In the project outlined here, I propose the analysis of the neuronal circuits involved in the generation of motor output, a behavior representing the ultimate output of nearly all nervous system activity. Studying the mouse motor output system will allow the analysis of neuronal circuit connectivity at an exquisite degree of specificity. Owing to the anatomical arrangement of motor neuron pools innervating individual muscles, this system offers the possibility to combine genetic, anatomical and physiological analysis of synaptic specificity with a direct link to a behavioral output. Generation of coordinated motor behavior is functionally linked to the high degree of specificity in presynaptic connections controlling the activation of individual motor neuron pools, yet knowledge on the specificity map of premotor circuits is currently missing. The aim of this research project is to acquire information on the general principles guiding the acquisition, maintenance and developmental plasticity of neuronal connectivity between premotor neurons and functionally defined subpopulations of motor neurons. This project is now possible due to the unique combination of our detailed know-how of the motor system in mice including a variety of genetic animal models, and the application of novel viral circuit tracing technology revealing monosynaptically connected premotor neurons, which we have recently applied successfully to the motor system in mice in vivo. Together, our project will elucidate the anatomical connectome of the motor output system as well as the principles governing the specificity with which motor circuits assemble.