The molecular chaperone Hsp90 is not only of major current interest in fundamental biological research but also recognised as an exciting new target for the treatment of cancer and other diseases. In addition to playing an important role in response to proteotoxic heat shock and others stresses, Hsp90 is also critical for maintaining normal cellular homeostasis. Hsp90 is responsible for ensuring the conformational stability, shape and function of a selected range of key proteins, including many kinases and transcription factors. Furthermore, recent studies show that Hsp90 plays a key role in development and evolution. Hsp90 is overexpressed in cancer cells and is thought to be involved in dealing with the cellular stress associated with malignancy, as well as being essential for a range of key oncogenic proteins, including ErbB2, Raf-1, Akt/PKB, mutant p53 and many others. A major attraction of Hsp90 inhibitors is their potential to inhibit a range of 'mission critical' cancer pathways, thereby blocking all of the 'hallmark traits' of malignancy and exhibiting broad-spectrum antitumour activity. The first-in-class Hsp90 inhibitor 17AAG has entered clinical trials with promising early results and a range of other agents is under investigation and preclinical development. This article reviews the current status and future prospects for the exploitation of Hsp90 as a new molecular target for cancer treatment.