Allergen-specific immunotherapy is the only curative treatment of allergies against insect venom, house dust mites, tree/grass pollens, or cat dander. Subcutaneous immunotherapy is successful to reorient the immune system and re-establish long-term tolerance. However, major drawbacks for using this route include: repeated injections, as well as the risk of anaphylaxis. In this context, alternative mucosal routes of administration are being considered together with the combined use of adjuvants/vector systems and recombinant allergens or peptide fragments. Herein, we review the current status in the use of mucosal routes (i.e., sublingual, oral, intranasal) for allergen-specific immunotherapy, as well as the latest understanding with respect to underlying mechanisms of action.