Curcumin is the active ingredient in turmeric, a traditional herbal remedy and dietary spice. Curcumin has numerous beneficial medicinal properties, including cancer chemopreventive and cancer chemotherapeutic activity. Curcumin displays complex redox activity and functions as both pro- and antioxidant. These opposing activities are observed in cell-free systems, cultured cells, and in intact organisms. The redox properties of curcumin are a key feature of its activity and are due to the inherent chemical activities of the molecule, as well as its ability to induce multiple signaling pathways. This chapter provides an overview of the in vitro and in vivo studies that links curcumin’s redox activity to its chemopreventive and chemotherapeutic effects. It also highlights the need for caution in combining curcumin with certain chemotherapies or in the setting of selected preexisting conditions.