Anthocyanins, plant pigments in fruits and berries, have been shown to delay cancer development in rodent models of carcinogenesis, especially those of the colorectal tract. Anthocyanins and anthocyanidins, their aglycons, especially cyanidin and delphinidin, have been subjected to extensive mechanistic studies. In cells in vitro, both glycosides and aglycons engage an array of anti-oncogenic mechanisms including anti-proliferation, induction of apoptosis and inhibition of activities of oncogenic transcription factors and protein tyrosine kinases. Anthocyanins and anthocyanidins exist as four isomers, interconversion between which depends on pH, temperature and access to light. Anthocyanidins are much more prone to avid chemical decomposition than the glycosides, and they only survive for minutes in the biophase. These pharmaceutical issues are very important determinants of the suitability of these flavonoids for potential development as cancer chemopreventive drugs, and they have hitherto not received adequate attention. In the light of their robust cancer chemopreventive efficacy in experimental models and their superior stability as compared to that of the aglycons, the anthocyanins seem much more suitable for further drug development than their anthocyanidin counterparts.