Since 1859, when Fordos reported the isolation of the blue pigment 5-N-methylphenazine-1-one, named pyocyanin, more than 100 different natural phenazines have been isolated and more of 6,000 compounds with a phenazinebased skeleton have been synthesised. The biological activities of these compounds including antimicrobial, antimalarial and antiparasitic activities have been reported, although since 1959, phenazines have been associated with anticancer activity and several publication and patents are available thus far. This chapter critically discusses the structural features of both natural and synthetic phenazines in relation to their in vitro, in vivo and available clinical anticancer activity along with a focus on the mode of action.