Among the most promising chemopreventive agents, certain natural polyphenols have recently received a great deal of attention because of their demonstrated inhibitory activity against tumorigenesis. In view of their anticancer properties, these compounds also hold great promise as potential chemotherapeutic agents. However, to translate these chemopreventive agents into chemotherapeutic compounds, their exact mechanisms of action must be delineated. By using a multidisciplinary approach guided by modern nuclear magnetic resonance spectroscopy techniques, fluorescence polarization displacement assays, and cell-based assays, we have begun to unravel the mechanisms of actions of certain polyphenols such as Gossypol (a compound from cotton seed extracts) and Purpurogallin (a natural compound extracted from Quercus sp. nutgall) and their derivatives. Our findings suggest that these natural products bind and antagonize the antiapoptotic effects of B-cell lymphocyte/leukemia-2 (Bcl-2) family proteins such as Bcl-x_L. Our in vitro and in vivo data not only open a window of opportunities for the development of novel cancer treatments with these compounds but also provide structural information that can be used for the design and development of novel and more effective analogues.