Multiple types of cells and cytokines are found that actively suppress immune responses, a function that is critical to maintain self-tolerance and immune homeostasis. Naturally occurring regulatory T cells (Tregs) and the pleiotropic cytokine transforming growth factor (TGF)-β are the best characterized. Dysregulation of either one leads to various immunopathologies under physiologic conditions, demonstrating their essential roles in immune suppression. Tregs and TGF-β play important roles in the development of lung-related immune disorders, such as asthma and allergy. Understanding the function and regulation of Tregs and TGF-β during immune responses offers therapeutic promise for the control of these diseases. Our laboratory has been interested in understanding the mechanisms of immune suppression, particularly in studying the interrelated functions of Tregs and TGF-β in immune regulation. In this article, we discuss the recent progress that we have made in the relevant areas.