Sublingual immunotherapy (SLIT) was first attempted more than a century ago. After a long parenthesis probably related to the lack of impressive clinical results, the advances on allergen quantification and characterization, together with the improvements in the recombination techniques have renewed the interest in this therapy during the past decade. There are currently enough high quality clinical trials on its efficacy in the management of respiratory allergies (asthma and rhinoconjunctivitis) to conclude that SLIT could be an effective tool for the management of those diseases. This effectiveness has been shown both in children and in adults. However, while there are some clues related to the mechanism of action of SLIT, there is still much to know about it. In addition, more studies comparing the effectiveness of SLIT vs the standard subcutaneous immunotherapy (SCIT) are needed to definitely establish the role of SLIT in the treatment of allergic diseases. SLIT has proven a very safe therapy as compared to SCIT, a fact which adds a very important advantage to the sublingual route.