Although allergen-specific immunotherapy (SIT) has been performed in humans for already a century, the immune mechanisms underlying this treatment are still not entirely solved. Allergen-specific CD4+ T lymphocytes are considered as pivotal for the induction and maintenance of allergic disorders. Consequently, their role for allergy treatment has been--and still is--of great interest. Whereas two decades ago immune deviation, i.e. the switch from the allergic Th2 response to a Th1-like response, was described as the most important alteration induced by SIT, more recently the induction of allergen-specific regulatory T cells producing IL-10 has been considered as a main event causing peripheral T cell tolerance. In view of very recent data indicating that both mechanisms may occur consecutively during allergy treatment this review summarizes the current understanding of the immunological mechanisms involved in allergy vaccination.