Patients presumed to have lower limb symptoms localizing to the lumbar or lumbosacral plexus require rigorous electrophysiological evaluation. Entities that cause lumbosacral plexopathies may be patchy, asymmetrical and more diffuse than initially suspected. As a result, bilateral nerve conduction studies and needle examination outside those routinely tested and clinically affected may be needed to document the extent of involvement including needle examination of the thoracic paraspinals and consideration of upper limb studies. This article outlines the lumbar and lumbosacral plexus anatomy, and discusses a differential diagnosis and electrophysiological approach in assessing patients with presumed lumbosacral plexopathies.