Allergen specific immunotherapy is the practice of administering increasing doses of an allergen to an allergic patient in order to reduce symptoms on subsequent exposure. The exact mechanism of action is unclear. It is thought to invoke a state of immune unresponsiveness among peripheral T-cells and a subsequent shift from the allergic type immune response to the non-allergic type. Immunotherapy is highly effective in reducing symptoms and medication need, and also improves objective measures of nasal allergy. Traditional subcutaneous administration can rarely lead to severe systemic reactions, but it is now being superseded by sublingual immunotherapy, which appears to be as effective but with less risk. This article reviews the immunological basis of immunotherapy and its clinical applications.