Intravenous immunoglobulin (i.v.Ig) has multiple actions on the immunoregulatory network that operate in concert with each other. For each autoimmune neuromuscular disease, however, there is a predominant mechanism of action that relates to the underlying immunopathogenetic cause of the respective disorder. The best understood actions of i.v.Ig include the following: (a) modulation of pathogenic autoantibodies, an effect relevant in myasthenia gravis (MG), Lambert-Eaton myasthenic syndrome (LEMS), Guillain-Barré syndrome (GBS), chronic inflammatory demyelinating polyneuropathy (CIDP), and stiff-person syndrome (SPS); (b) inhibition of complement activation and interception of membranolytic attack complex (MAC) formation, an action relevant to the complement-mediated mechanisms involved in GBS, CIDP, MG, and dermatomyositis (DM); (c) modulation of the inhibitory or activation Fc receptors on macrophages invading targeted tissues in nerve and muscle, as seen in CIDP, GBS, and inflammatory myopathies; (d) down-regulation of pathogenic cytokines and adhesion molecules; (e) suppression of T-cell functions; and (f) interference with antigen recognition. Controlled clinical trials have shown that i.v.Ig is effective as first-line therapy in patients with GBS, CIDP, and multifocal motor neuropathy (MMN), and as second-line therapy in DM, MG, LEMS, and SPS. In paraproteinemic IgM anti-MAG (myelin-associated glycoprotein) demyelinating polyneuropathies and inclusion body myositis (IBM), the benefit is variable, marginal, and not statistically significant. i.v.Ig has a remarkably good safety record for long-term administration, however, the following side effects have been observed: mild, infusion-rate-related reactions, such as headaches, myalgia, or fever; moderate but inconsequential events, such as aseptic meningitis and skin rash; and severe, but rare, complications, such as thromboembolic events and renal tubular necrosis. Future studies are needed to (a) find the appropriate dose and frequency of infusions that maintain a response; (b) address pharmacoeconomics, comparing the high cost of i.v.Ig to the cost of the other therapies, which, although less expensive, cause significantly more long-term side effects; (c) determine why some patients respond better than others; and (d) examine the merits of combining i.v.Ig with other immunosuppressive drugs.