The role of distinct CD4(+) T-cell populations in regulating the nature and strength of immune responses is well documented and in the past has principally focused on the cross-regulation of T-helper type 1 (Th1) and Th2 cells, which secrete interferon-gamma and interleukin-4, respectively. However, the identification of T cells capable of suppressing responses mediated by Th1 and Th2 cells, termed regulatory T cells (Treg cells), has prompted a paradigm shift in our understanding of the regulation of immune responses to infection and environmental antigens. This article focuses on the role of Treg cells in the lungs following infection with respiratory pathogens and discusses the targeting of Treg cells in the development of new therapies for immune-mediated respiratory diseases, such as allergy and asthma.