Allergen-specific immunotherapy, together with drug therapy and allergen avoidance, is a cornerstone in the management of respiratory allergy in both adults and children. Since the introduction of sublingual administration of immunotherapy (SLIT), it was regarded as the most promising approach, especially because of its optimal safety profile. After 15 years of clinical trials, the WHO officially accepted SLIT as a routine clinical option in both adults and children. This paper reviews the up-to-date aspects about clinical efficacy, safety and pharmacokinetics of sublingual immunotherapy.