Allergic diseases affect a large proportion of the population of the United States. Although there are many effective pharmacologic therapies available, only allergen-specific immunotherapy has been shown to have significant and long-lasting therapeutic and immunomodulatory effects for the management of allergic rhinitis, allergic asthma, and venom hypersensitivity. Allergen immunotherapy requires a build-up phase as the dose of the vaccine is increased until a therapeutic (maintenance) level is achieved. This maintenance dose is usually continued for 3 to 5 years. Most patients tolerate immunotherapy well, but local reactions are not uncommon. Immunotherapy should only be administered in a physician's office because some patients may experience systemic anaphylactic reactions requiring immediate therapy. Even with newer therapies on the horizon, allergen immunotherapy will continue to have an important role in the treatment of allergic diseases.