The incidence of allergic diseases is increasing at an alarming rate, paricularly in countries with a western lifestyle. Currently in the UK, approximately one quarter of the population suffers from seasonal allergic rhinitis. Most patients can be treated with conventional pharmacotherapy on an ‘as needed’ symptomatic basis whereas allergen immunotherapy represents a useful treatment approach in carefully selected patients with severe, IgE-mediated disease. Allergen immunotherapy can deliver improvements in hayfever symptoms over and above that which can be achieved by pharmacotherapy. In addition, unlike pharmacotherapy, allergen immunotherapy provides long-term clinical benefits. These include long-term disease remission, prevention of new atopic sensitisations and a reduction in disease progression from rhinitis to asthma. This review provides a comprehensive update on the mechanisms of allergen injection immunotherapy, recent data on the mechanisms of sublingual allergen immunotherapy is also included.