Specific immunotherapy (IT) introduced in 1911 for the treatment of asthma is still controversial, since many protocols have been devised empirically, some extracts are still poorly defined, the mechanisms of action are not yet clear and allergen injections can cause serious side effects. However, clinical efficacy of allergen-specific IT in hay fever with seasonal asthma and in selected patients with perennial asthma has been convincingly documented. For optimal effectiveness IT should be restricted to appropriately selected patients with proven IgE-mediated disease. IT has seldom the capacity to act as a curative treatment. However, a considerable reduction in symptoms and drug consumption is of sufficient benefit. The clinical efficacy of IT should not be solely compared to symptomatic therapy, since IT should be considered as a part of a multifactorial approach aimed at interfering broadly with the pathophysiology of allergic diseases.