Sublingual immunotherapy (SLIT) is a viable alternative to the subcutaneous route for the treatment of respiratory allergy, whereas the pure oral route has been abandoned because of its lack of efficacy. The main distinctive feature of SLIT is its optimal safety profile, which has been demonstrated in adults and children. The indications for SLIT are similar to those for the subcutaneous route. A long-lasting effect has been demonstrated for the sublingual route, but data are needed to determine the optimal dose and the preventive effect in asthma.