Allergen immunotherapy is effective for the treatment of allergic rhinitis, allergic asthma and Hymenoptera venom allergy. In view of potential side effects, cost, and the necessary patient commitment, an important question is whether allergen immunotherapy provides persistent clinical benefits after treatment discontinuation. Here we appraise the existing evidence for long-term effects of both subcutaneous and sublingual immunotherapy in terms of clinical efficacy, immune mechanisms, prevention of asthma development and prevention of new allergen sensitisations. Evidence from large, randomised, double-blind, placebo-controlled clinical trials that include a follow-up phase after treatment cessation demonstrate long-term efficacy. The data strongly support recommendations in international guidelines that both sublingual and subcutaneous immunotherapy should be continued for a minimum of 3 years to achieve disease modification and long-term tolerance. Grass pollen immunotherapy for seasonal rhinitis may inhibit the onset of asthma symptoms and requirements for asthma medication. Whether early intervention in infancy with mite sublingual immunotherapy may prevent asthma remains to be tested.