Publisher Summary This chapter explains some of the key issues regarding the structure and function of the developing corticospinal tract. Abnormal development of descending motor systems is associated with a variety of movement disorders. In primates, some corticospinal neurones establish a monosynaptic linkage between the primary motor cortex and spinal motoneurones, particularly those innervating hand and finger muscles. The use of electrophysiological approaches to prove the existence of a monosynaptic connection that requires intracellular recording from the target motoneurones. The monosynaptic origin of the excitatory postsynaptic potential (EPSP) is indicated when the segmental delay between the arrival of the tract volley and EPSP onset is too short to involve more than one synapse. The chapter also examines the change in axon diameter of the corticospinal neurons change during development and its relation to conduction velocity. The myelination of corticospinal axons is a postnatal process clearly protracted with respect to that of the other descending pathways. This period of myelination far outlasts that in which the spinal gray matter receives corticospinal innervation. In primates, corticospinal axons seem to be myelinated over their cranial before their spinal course. In the spinal cord, myelination of the corticospinal tract follows a rostral-to-caudal gradient. The chapter also elaborates the use of transcranial magnetic stimulation for studying corticospinal development.