The transforming growth factor β (TGF-β) plays a dual role in allergic disease. It is important in suppressing T cells and also mediates repair responses that lead to unwanted remodeling of tissues. Advances in the immunology of allergy indicate that allergens cause overreactions in the lymphocyte compartment because of the lack or decreased number of suppressive, regulatory T cells. TGF-β was shown to induce regulatory T cells and participate directly in suppression of effector T cells. Therefore, TGF-β may help return reactivity to allergens to normal subsymptomatic activity. Whether chronic inflammatory diseases such as asthma profit from TGF-β–mediated suppression of specific immune responses or whether the TGF-β–mediated tissue remodeling aggravates diseases more than it helps control immune reactions is unclear. This article addresses these issues and future strategies in this field.