Motor unit number estimation (MUNE) techniques have evolved substantially over the past decade and have been applied, with increasing frequency, to the study of amyotrophic lateral sclerosis (ALS). As major clinical therapeutic trials in motor neuron disease continue to appear, the need for a clear quantitative method of following motor unit physiology in the living subject grows ever more pressing. In this article, we review the major modern techniques of MUNE and the data supporting their reproducibility and utility in patients with ALS, with particular attention to their role in evaluating the efficacy of new therapeutic interventions.