The 90 kDa heat shock protein, hsp90, is a major molecular chaperone of the cell that appears to have particular significance to cellular regulatory processes. New tools and approaches have revealed a number of target proteins for hsp90, most of which ave protein kinases or transcription factors. While the mechanism of action of hsp90 is not well understood reasonable models have emerged describing some functional domains of this protein, the importance of conformational transitions for its activity and its role within a multi-component chaperoning pathway of the cell.