Although allergen immunotherapy (AIT) has been used for the treatment of allergic rhinitis (AR), allergic conjunctivitis, asthma, stinging insect hypersensitivity, and atopic dermatitis for over 100 years, it has been slow to gain universal acceptance. With the publication of the "World Health Organization Position Paper, Allergen Immunotherapy" in 1998 which summarized the scientific evidence for the efficacy and long-term benefit of this therapy, it has become an accepted and respected modality of treatment. In this review there are discussed following topics of allergen immunotherapy: the reasons for recommending AIT, mechanism of action, subcutaneous and sublingual methods of application, duration, adherence and cost effectiveness. It is necessary to support, not blame, the patient for nonadherence as it is the responsibility of the patient, the physician, and the health-care system to create an environment in which the patient can be adherent. Nonadherence is multifactorial in most every patient and the physician must address all of the factors if adherence is to be improved. The greatest challenge is taking the time to create an individualized patient-tailored strategy to improve adherence, as one size does not fit all. Adherence is dynamic and selecting the best time to start AIT and assuring that there is close follow-up through AIT years of treatment is essential. The patient who is persistent and consistent in year one of AIT may not continue to be so in year three without added encouragement and support. The health-care systems and professional organizations need to help train physicians and their staff both in efficient and accurate ways to assess nonadherence and in implementing interventions to optimize adherence. The multidisciplinary approach to treating this disease of nonadherence will require the involvement of all healthcare professionals, researchers, professional organizations, insurance companies, and policy-makers.