Naturally occurring regulatory T cells (Tregs) have been shown to suppress immune responses to self-antigens, thereby limiting autoimmunity. In the case of tumours, where immune responses to self-antigens are beneficial and lead to elimination of the tumour, such suppressive activity is actually detrimental to the host. Manipulation of Tregs holds great promise for the immunotherapy of cancer. Several studies performed using rodent models and indicate that Tregs cells inhibit effective anti-tumour immune responses and that their removal promotes tumour rejection. The increasing number of studies of Tregs in patients with cancer also point to a role for these cells in promoting disease progression. This review summarises the findings of these studies and addresses the advantages and potential pitfalls of manipulating Treg activity for the treatment of cancer.