Present knowledge regarding the clinical efficacy and safety of sublingual immunotherapy (SLIT) for the treatment of respiratory allergy is reviewed. Allergen-specific immunotherapy is presently considered a 'biological response modifier' for the treatment of respiratory allergy, to be used in association with drug therapy and allergen avoidance. Its value in the treatment of these conditions has been established in position papers from the World Health Organization and the European Academy of Allergology and Clinical Immunology. Immunotherapy is usually administered subcutaneously (SCIT), and with this route several severe adverse events and fatalities have been described. Therefore, in the last 15 years, novel and safer routes of administration (local routes) have been developed. SLIT, in particular, has been investigated in 18 randomised controlled clinical trials. SLIT's clinical efficacy (improvement in symptoms and reduction in drug intake) in both asthma and rhinitis has been clearly assessed in 16 of these studies and for the most common allergens. SLIT's safety profile, derived from the clinical trials and from post-marketing surveillance studies, was shown to be satisfactory in both adults and children. The most frequently reported adverse events are gastrointestinal complaints, which can be avoided through appropriate dosage adjustment. For these reasons, SLIT has been accepted as a viable alternative to SCIT in recent position papers. The main advantages of SLIT are its safety (no severe systemic adverse event has ever been described) and good patient acceptance, especially in children; in addition, SLIT is a self-administered treatment that can be carried out at home by the patient. In contrast to injection immunotherapy, knowledge of the mechanisms of action of SLIT is still developing, albeit rapidly, although interesting data about its pharmacokinetics in humans are available. Data are also required concerning the possible preventive and long-lasting effects. SLIT is a viable alternative to SCIT, with the same rationale and indications. It is intended to be used in association with proper pharmacological treatment, at the earliest stages of the disease, for optimal management of respiratory allergy.