Cancer control by adaptive immunity involves a number of defined death(1-8) and clearance(9-11) mechanisms. However, efficient inhibition of exponential cancer growth by T cells and interferon-gamma (IFN-gamma) requires additional undefined mechanisms that arrest cancer cell proliferation(1-5,12,13). Here we show that the combined action of the T-helper-1-cell cytokines IFN-gamma and tumour necrosis factor (TNF) directly induces permanent growth arrest in cancers. To safely separate senescence induced by tumour immunity from oncogene-induced senescence(9-11,14-17), we used a mouse model in which the Simian virus 40 large T antigen (Tag) expressed under the control of the rat insulin promoter creates tumours by attenuating p53- and Rb-mediated cell cycle control(18,19). When combined, IFN-gamma and TNF drive Tag-expressing cancers into senescence by inducing permanent growth arrest in G1/G0, activation of p16INK4a (also known as CDKN2A), and downstream Rb hypophosphorylation at serine 795. This cytokine-induced senescence strictly requires STAT1 and TNFR1 (also known as TNFRSF1A) signalling in addition to p16INK4a. In vivo, Tag-specific T-helper 1 cells permanently arrest Tag-expressing cancers by inducing IFN-gamma- and TNFR1-dependent senescence. Conversely, Tnfr1(-/-) Tag-expressing cancers resist cytokine-induced senescence and grow aggressively, even in TNFR1-expressing hosts. Finally, as IFN-gamma and TNF induce senescence in numerous murine and human cancers, this may be a general mechanism for arresting cancer progression.