Allergen immunotherapy involves exposing a patient to a gradually escalating dose of a specific allergen with the intention of decreasing allergic and inflammatory responses, ultimately leading to a sustained decrease in allergic symptoms. A build-up phase (once weekly injections) is followed by a maintenance phase (once monthly injections) that generally continues for 3 to 5 years. Allergen immunotherapy is indicated for select patients with allergic rhinoconjunctivitis, allergic asthma, and stinging insect hypersensitivity. The safety and efficacy of allergen immunotherapy have been confirmed by numerous well-designed studies. Recent research has helped uncover the mechanisms by which allergen immunotherapy exerts its therapeutic effect, paving the way for the development of safer, more effective therapy for a wider range of allergic diseases.