Regulatory cells comprise a highly heterogeneous T-cell population. It is assumed they are generated both in the thymus and periphery. According to some immunologists, any T-cell population has the potential to differentiate into cells with regulatory properties in the periphery. Following the discovery of their role in maintaining T-cell homeostasis and their ability to prevent autoimmune disease, allergy, and other types of hypersensitivity, regulatory cells have been the focus of intense studies over the last decade. During pregnancy, regulatory cells are believed to mediate tolerance to the fetus. In the unnatural event of transplantation, regulatory cells induce and maintain tolerance to the transplanted tissues or cells and prevent graft-versus-host disease. At the same time, however, regulatory cells are responsible for decreased immunity against tumors and infections. The data which have emerged to date concerning regulatory cells have not allow them to be definitively identified nor their characteristics to be clearly defined. This may be due to the existence of various regulatory cell populations as well as different modes of action depending on the experimental model. Contradictory in vitro and in vivo results and lack of certainty as to how the results of animal trials correspond with the human model are why the notion of harnessing regulatory cells as therapeutic agents is still a distant one, although raising great hopes. The aim of the following article is to make sense of the patchwork of available data concerning regulatory cells and review the potential use of these cells in the field of transplantation.