The peroxisome proliferator-activated receptor γ (PPARγ) ligands have anticancer activity against a wide variety of neoplastic cells in vitro. Animal studies have chronicled their in vivo anticancer effects and chemopreventive capabilities. In addition, moderate anticancer activities of PPARγ ligands with minimal toxicities have been observed in patients with liposarcomas and prostate cancers. These compounds can slow growth and induce partial differentiation of selected cancer cells. They can decrease levels of cyclin D₁ and E, inflammatory cytokines, and nuclear factor κB and increase expression of p21^waf1 and p27^kip1. Surprisingly, some or many of these effects may occur independently of PPARγ. Other data suggest that PPARγ may behave as a tumor suppressor gene, although several compelling murine models, paradoxically, suggest that under selected circumstances, PPARγ ligands may stimulate cancer formation. Nevertheless, the bulk of studies showed that PPARγ ligands do have antiproliferative activity against many transformed cells and may be helpful in the setting of adjuvant and chemopreventive treatments of several common tumors, including colon, prostate, and breast cancers.