Regulatory T cells, which play an important role in the maintenance of the immune tolerance and immune homeostasis, have become a hot research field in immunology in recent years. Researching on the regulatory T cell is not only very important to realize the complex immune system, but also has the great potential application to the therapy of autoimmune diseases, cancer, HIV infection and transplantation. The recent experimental results have demonstrated that a transcription factor Foxp3 plays a critical role in the development of regulatory T cell and is a critical regulator for the development of regulatory T cells.