Natural products have continued to drive the development of new chemotherapeutics and elucidation of new biological targets for the treatment of disease. Since Whitesell and Neckers' original discovery that geldanamycin does not directly inhibit v-Src, but instead manifests its biological activity through inhibition of the Hsp90 molecular chaperone, additional natural products and natural product derivatives have been identified and developed to inhibit the Hsp90 protein folding machinery. 17-AAG, a geldanamycin analogue, is currently in clinical trials for the treatment of several types of cancer. Recent work has produced improved radicicol analogues that show promising Hsp90 inhibitory activity in vitro. In addition, chimeric molecules of these two natural products are active in vitro and represent a novel class of Hsp90 inhibitors for cancer treatment. In addition to their chemotherapeutic uses, natural product inhibitors and their derivatives have been utilized to probe the biological mechanisms by which Hsp90 inhibition regulates tumor cell growth. As a consequence of these studies, the molecular chaperones have emerged as an exciting new class of therapeutic targets. This review will highlight the utility of the natural products, geldanamycin and radicicol, as well as improved analogues and the activities exhibited by these compounds against various cancer cell lines.