Defining the role of sublingual immunotherapy (SLIT) for the treatment of allergic rhinoconjunctivitis and allergic asthma is hampered for various reasons: Heterogeneity in study designs, different allergen extracts and dosages, imperfect assessment strategies and partially inconclusive results. A number of questions need to be addressed before replacing subcutaneous immunotherapy (SCIT) by the sublingual route: Ideal dose, treatment duration, magnitude of improvement, modification of the immune response, long-term and preventive effects. At present, SLIT might be used in adults with pollen related rhinoconjunctivitis, particularly if SCIT is not suitable for the patient (i.e. systemic effects). Only few data support SLIT for house dust mite allergy or bronchial asthma. Due to a lack of convincing results SLIT for children should only be applied in controlled studies and not in the daily routine. A more substantiated and conclusive judgment of SLIT is possibly warranted in a few years, when more studies with larger patient groups have addressed open questions concerning SLIT.