Increasing evidence supports the existence of elevated numbers of regulatory T cells (T_reg cells) in solid tumors and hematologic malignancies. Whereas the biology of CD4⁺CD25⁺FOXP3⁺ T_reg cells in murine models seems to be rather straightforward, studies in human diseases are more difficult to interpret due to expression of CD25 on activated effector T cells as well as T_reg cells. More importantly, early studies in human tumors were mainly focused on CD4⁺CD25⁺ T_reg cells lacking interrogation of more specific markers such as FOXP3 expression. Although the increase of T_reg cells seems to be a characteristic feature in most tumors, little is known about the molecular and cellular mechanisms responsible for the increase and maintenance of elevated levels of T_reg cells in cancer. We will discuss earlier data in the context of recent findings in T_reg-cell biology with a particular emphasis on CD4⁺CD25^highFOXP3⁺ T_reg cells in human malignancies.