Regulatory T (T reg) cells play key roles in various immune responses. For example, T reg cells contribute to the complex pathogenesis of inflammatory bowel disease (IBD), which includes Crohn’s disease and ulcerative colitis during onset or development of that disease. Many animal models of IBD have been used to investigate factors such as pathogenic cytokines, pathogenic bacteria, and T-cell functions, including those of T reg cells. In addition, analyses of patients with IBD facilitate our understanding of the precise mechanism of IBD. This review article focuses on the role of T reg cells and outlines the pathogenesis and therapeutic strategies of IBD based on previous reports.