Organ transplantation is a preferred treatment option for patients with end-stage organ failure. However, transplant induces a robust rejection response that necessitates life-long immunosuppression, which often leads to a plethora of comorbidities. Thus, the goal of transplantation is to achieve a state of tolerance wherein the host permanently accepts the transplanted organ while maintaining normal immune responses to other antigens. Regulatory T cells (Tregs) play an important role in realizing this goal and are being explored in both animal models and human clinical trials. In this chapter, we discuss the key principles of transplant rejection and Treg biology, as well as the status of human clinical trials utilizing Tregs as cellular therapy. We discuss how the current immunosuppressive drugs are utilized in transplantation in favoring an increased Treg to T effector cell ratio, different approaches in generation of therapeutic Tregs, and various facets in Treg trial designs in the clinic. Such clinical trials provided many opportunities to leverage our understanding of Tregs in transplantation. They also demonstrated Tregs as a safe cellular therapy for human use, but the efficacy of this treatment has yet to be fully realized.