Guillain-Barré syndrome (GBS) is an acute inflammatory polyradiculoneuropathy, which has various clinical presentations and both axonal and demyelinating forms. The original description of “ascending paralysis” encompasses the most common varieties: the primary demyelinating form, acute inflammatory demyelinating polyneuropathy (AIDP), and some of the axonal forms, acute motor axonal neuropathy (AMAN) and acute motor and sensory axonal neuropathy (AMSAN). However, there are now well-documented acute “monophasic” polyneuropathies that have a different clinical phenomenology than that described originally by Guillain, Barré, and Strohl: Miller Fisher syndrome, pure sensory neuropathy/neuronopathy, pandysautonomia, and oropharyngeal variant. Here the authors review both typical GBS (AIDP, AMAN, and AMSAN), and variant syndromes with a focus on clinical and diagnostic features, pathologic findings, pathogenesis, and treatment.