Intravenous immunoglobulin is used as a replacement therapy in primary immunodeficiency diseases as well as an immuno-modulatory agent in a variety of autoimmune and inflammatory disorders. The mechanisms of intravenous immunoglobulin action are complex and, for some disorders, not well understood. This paper reviews the recent literature and discusses approved, new, and controversial indications for intravenous immunoglobulin therapy, with special emphasis on its mechanism of action. Curr Opin Pediatr 11533–539© 1999 Lippincott Williams & Wilkins, Inc.