The 90 kDa heat shock protein Hsp90 is a highly conserved and very abundant protein in the cytosol of both eukaryotic and prokaryotic cells. The main focus in the recent years has been concerned Hsp90's interaction with untransformed steroid receptors and newly synthesized kinases. Within these heterocomplexes, Hsp90 acts in concert with several other heat shock and non heat shock proteins to mediate important regulatory effects. These roles of Hsp90 leave unexplained its high abundance and heat shock regulation. More recently, however, Hsp90 has been identified as an ATP independent molecular chaperone, which binds transiently to folding intermediates in vitro, prevents aggregation and supports the refolding of the intermediates to the native state. The finding that Hsp90 interacts with late, probably highly structured, folding intermediates led to the suggestion that Hsp90 might function as a general chaperone for well structured not yet native polypeptides. This explanation provides the missing link between Hsp90 on the one hand as a highly specialized binding protein and Hsp90 on the other hand as a rather promiscuous molecular chaperone.