CD4+CD25+FOXP3+ suppressive regulatory T cells (Treg) represent a subset of immune regulatory cells. Based on experimental results, Treg have recently been considered as a potential treatment option in several diseases. Compared with murine Treg, human CD4+CD25+FOXP3+ cells are less well characterized and understood, so a thorough understanding of their biology is vital before clinical applications can be initiated. This review summarizes knowledge on generation, phenotypic characteristics and function of human Treg. The possible role of these cells in organ transplantation, as well as interactions between immunosuppression and Treg are also discussed.