The lack of medication for allergy symptoms at the end of the last millennium has been the promoter of the idea of treating allergies as if you were treating an infectious disease, by vaccination prophylaxis. Two forms of AIT 1) subcutaneous immunotherapy (SCIT) and 2) sublingual immunotherapy (SLIT) are used in the world. Considerable interest has emerged in SLIT both scientifically and especially financially. SLIT is not a new treatment modality. First description dates back to 1900 when H. Curtis. It was relatively widely used until the late 1970's mainly in US by homeopathic therapists.A number of case series describing experience with the oral route were published during the 1920s and 1930s, but it seems to have been perceived not as efficacious nor as well tolerated as subcutaneous immunotherapy. The companies producing allergen immunotherapy have an alliance with important opinion leaders on both shores of the Atlantic.If SLIT did not work for 40 years, why should it work for respiratory allergic diseases today? This question is the mother of all questions in the field of respiratory allergic diseases. The purpose of this chapter is to provide past and current information about immunotherapy, and discuss controversies over efficacy and safety, and dosing considerations for SLIT to grass.