Sublingual immunotherapy (SLIT) is a matter of only 20 years. Nonetheless, in this short period of time more than 60 randomized double blind placebo-controlled trials have been published, in addition to postmarketing surveillance studies and meta-analyses. The wide diffusion of SLIT in clinical practice and the large availability of experimental data prompted the WAO to publish a position paper on SLIT, to identify the indications, contraindications, and practical aspects of the treatment. On the basis of the available literature, SLIT is certainly indicated in allergic rhinitis in both adults and children. In this latter population, SLIT may exert a preventative effect on the development of asthma. The age seems not to represent a special problem. SLIT can be used also when asthma is associated to rhinitis, whereas it is not the first choice for the treatment of isolated asthma. The IgE-mediated mechanism and the clear identification of the causal role of the allergen are mandatory prerequisites for prescribing SLIT. The safety profile is excellent, but it is recommended that the first dose be given under medical supervision. Atopic dermatitis, latex allergy, and hymenoptera hypersensitivity are promising fields of use of SLIT, but they are still considered only experimental uses.