Allergen immunotherapy is a more than 100-year-old treatment in particular for birch pollen, grass pollen, house dust mite and cat dander sensitised allergic patients. The mechanism of allergen-specific immunotherapy is complex. Different hypotheses have been postulated to explain the mode of action, such as a decrease of the number of tissue mast cells, eosinophils and basophils, an increase of IgG4 and IgA synthesis, a shift from Th2 to Th1 cells and an increase in the number and function of IL-10 producing T-regulatory cells (T-reg). All these immunological effects may contribute to immune tolerance and long-term changes in the immune system. The efficacy and safety of subcutaneous (SCIT) and sublingual immunotherapy (SLIT) with pollen and house dust mite have been investigated in many trials, meta-analyses and reviews. Nowadays grass pollen SLIT and SCIT, and birch pollen and house dust mite SCIT are implemented in clinical practice to treat therapy-resistant patients. However, the treatment is not effective for all patients and often not without side effects. Therefore, the development of new, safer and more effective immunotherapies is needed. These are approached along novel routes, including improved administration, combined treatment with immune response modifiers, fusion with immune response modifiers, allergen coupled to adjuvants and reconstruction of natural extracts with multiple recombinant allergens or with modified allergens. These developments are promising, but more research is required to implement them in clinical practice.