Allergen immunotherapy (AIT) was introduced in clinical practice more than 100 years ago. The clinical effectiveness in allergic rhinitis (and asthma) and in hymenoptera allergy was apparent early on but it was not until the mid-1900s that randomized placebo-controlled trials proved its efficacy. In the 1980s, sublingual immunotherapy (SLIT) was accepted in official guidelines. The availability of safer routes, such as SLIT, prompted increasing investigation of AIT for food allergy. The introduction of molecular-based diagnosis introduced the possibility of better targeted prescription of AIT. Other approaches are being explored, such as immunogenic peptides, recombinant allergens, and adjuvants.