Current pharmacological therapies for allergic diseases can improve clinical symptoms but cannot change their long-term clinical course. There is an unmet need for a curative treatment for allergic diseases. Allergen immunotherapy (AIT) is the practice of administering increasing doses of clinically relevant allergens to an allergic subject to reduce the clinical symptoms associated with subsequent exposure to the allergen. AIT is clinically effective for allergic asthma, allergic rhinitis, venom-induced anaphylaxis, and atopic dermatitis. AIT can change the natural course of allergic diseases and induce allergen-specific immune tolerance. In current clinical practice, AIT is delivered either subcutaneously or sublingually. Both subcutaneous and sublingual AIT have long-term therapeutic efficacy after of 3-5 years of treatment. The development of safer and more effective AIT strategies is needed. Conclusion: AIT is a disease-modifying therapy for allergic diseases. Future development of AIT should be directed toward achieving long-term clinical remission in patients with allergic diseases by the safe and effective induction of immune tolerance.