Since its first description in 1916, the spectrum of Guillain-Barre syndrome (GBS) has broadened and it is currently used to refer to a group of acute autoimmune neuropathies with different clinical phenotypes. Its spectrum is dominated by the classical phenotype characterized by flaccid paraplegia or tetraplegia with a mostly ascending course.In this book, we have presented in a simplified way the basics of neurophysiology applied to the clinic, the pathophysiology of GBS and the contribution of the electroneuromyogram in case of clinical suspicion of GBS.