Hsp90 is a chaperone that is critically important for both cancer progression and tumor survival. Hsp90 is an exciting target for anti-cancer drugs because most of the proteins that interact with Hsp90 are known to be in the cell cycle, signaling and chromatin-remodeling pathways. Recent work in fungi has shown that reduction of Hsp90 activity dramatically increases the efficacy of many fungicides. Furthermore, in studies on the evolution of drug resistance in fungi, it has been shown that high levels of Hsp90 increase the rate of the development of fungicide resistance, whereby low levels of Hsp90 decrease the rate of fungicide resistance. Similarly, in humans and mammalian models, Hsp90 inhibitors have been shown to act additively or synergistically with many other cancer therapies for killing both solid tumors and leukemias. Also, several recent studies have shown that Hsp90 inhibitors potentiate the activity of drugs in cancer cells lines that are otherwise resistant to the drug. However, during the evolution of drug resistance in cancer cells, it has not yet been determined whether early exposure to Hsp90 inhibitors slows the rate of developing resistance to other anti-cancer drugs, as would be expected from the fungal studies. In this review, we summarize the effects of the Hsp90 inhibitors geldanamycin and its derivatives with other anti-cancer drugs on killing cancer cells. We also discuss other basic science and clinical studies that need to be done to determine the optimum exposure regimens for Hsp90 inhibitor treatments to maximize its cancer-killing activities, and to minimize the evolution of resistance to other anti-cancer drugs.