Over the last years a number of reports have described elevated numbers of regulatory T (Treg) cells inside of tumors, in close proximity of the tumor, draining lymph nodes and also in peripheral blood of patients with solid tumors and hematologic malignancies. There is increasing evidence that Treg cells can migrate into tumors and suppress effective anti-tumor responses in the tumor microenvironment, thus contributing to the prosperity and growth of human tumors. In addition, several mechanisms have been described how conversion of conventional CD4+ T cells into Treg cells can occur in the context of human tumors, yet little is known about the molecular and cellular features responsible for the increase and maintenance of elevated levels of Treg cells in cancer. Recent studies now have elucidated how Treg cells mediate regulatory activity in the tumor microenvironment and enhanced our understanding of the underlying molecular mechanisms. Targeting Treg cells therefore provides an attractive therapeutic strategy to potentially influence the suppressed immune response in tumor patients thereby altering and supporting anti-tumor therapy.