This book aims to provide a comprehensive and timely review of new findings in motoneuron research. Recent findings have revealed that motoneurons are more complex and have more extensive functions than previously imagined. Some of the molecular and genetic pathways that orchestrate the development of motoneurons have been discovered, as have the mechanisms responsible for the selective innervation of muscles by specific pools of motoneurons. These novel findings are revolutionizing ideas about the function of motoneurons and have important implications for motoneuron disease.