Electrophysiologic assessment is an important diagnostic tool in motor neuron diseases, contributing on occasions to define a patient's prognosis and clinical course as well as to further our understanding of the disease itself. The advent of motor cortex stimulation techniques facilitates the study of impaired descending motor pathways in predominantly upper motor neuron forms of amyotrophic lateral sclerosis. The most important feature of this technique is the decrease in amplitude of muscle responses to cortical stimuli, as a reflection of upper and lower motor neuron death. Coaxial needle electromyography (EMG) affords data on muscle fiber denervation as well as alterations in motor unit potentials, whose amplitude and duration increase. Motor and sensory nerve conduction velocities are preserved: single fiber and macro-EMG studies reveal changes suggestive of collateral sprouting of distal axons in surviving motor units in a compensatory reinnervation process. None of these techniques, however, provides a direct measure of the number of motor units present in a muscle, information that would be an essential aid in following the natural course of such diseases as well as assessing the efficacy of experimental treatments. Several techniques for estimating the number of motor units have been put forth, although none has been widely accepted. We describe the techniques proposed, weighing the advantages and disadvantages of each. At present it is impossible to tell which technique best estimates the real number of motor units present.