Hsp90 is a molecular chaperone associated with the folding of signal-transducing proteins, such as steroid hormone receptors and protein kinases. Results from recent studies have shed light on the structure of Hsp90 and have demonstrated that it can bind to and hydrolyse ATP. Hsp90 forms several discrete subcomplexes, each containing distinct groups of co-chaperones that function in folding pathways. Although Hsp90 is not generally involved in the folding of nascent polypeptide chains, there is a growing list of proteins whose activity depends on its function, including heat-shock factor. This review addresses recent developments in our understanding of the structure and function of Hsp90.