The 90-kDa family of heat shock proteins are major molecular chaperones found in bacteria, mammals, yeast and plants with high degree of conservation. The cytosolic Hsp90 and the endoplasmic reticulum-located Grp94 (94-kDa glucose regulated protein) are the best-characterised representatives. Hsp90 appears to have an important role in the folding and activation of proteins involved in signal transduction and control of the cell cycle, whereas Grp94 has been implicated in the correct folding and assembly of proteins passing through the ER. Until recently the mechanism of action of this important group of proteins was one of the least well understood of the molecular chaperones, but significant progress has been made in understanding its biochemistry. Dissection of the functional domains of the protein has revealed the mechanism of action as well as the importance of conformational transitions for its activity. This review seeks to provide an overview of current knowledge of the structure and function of the Hsp90 family of proteins with particular emphasis on their role in plant cells.