Regulatory T cells (Treg) provide protection from autoimmune disease, graft-versus-host disease, transplant rejection and overwhelming tissue destruction during infections. Conversely, high Treg numbers enable cancer cells to evade the host immune response. Thus, Treg are seen as an important tool to manipulate the immune response. However, as the immunological community is trying to move this knowledge from mice to humans, contradictory results regarding the number and function of Treg in various diseases are appearing. This problem arises because we cannot clearly define Treg populations on the basis of expression of CD25 and other cell surface markers in humans. This review addresses the utility of the FOXP3 forkhead transcription factor for the identification of Treg populations and summarizes recent data on the expression of FOXP3 in lymphomas. It is crucial to really understand Treg biology before attempting therapies, including (i) the injection of expanded Treg to cure autoimmune disease or prevent graft-versus-host disease or (ii) the depletion or inhibition of Treg in cancer therapy. For instance, new data arising from the study of haematological malignancies highlight the additional complexity of systems where malignant cell populations may also be direct Treg targets.