Molecular chaperones facilitate the correct folding of other proteins under physiological and stress conditions. Recently it has become evident that various co‐chaperone proteins regulate the cellular functions of these chaperones, particularly Hsp70 and Hsp90. Hop is one of the most extensively studied co‐chaperones that is able to directly associate with both Hsp70 and Hsp90. The current dogma proposes that Hop functions primarily as an adaptor that directs Hsp90 to Hsp70‐client protein complexes in the cytoplasm. However, recent evidence suggests that Hop (...) can also modulate the chaperone activities of these Hsps, and that it is not dedicated to Hsp70 and Hsp90. While the co‐chaperone function of Hop within the cytoplasm has been extensively studied, its association with nuclear complexes and prion proteins remains to be elucidated. This article will review the structural features of Hop, and the evidence that its biological function is considerably broader than previously envisaged. BioEssays 26:1058–1068, 2004. © 2004 Wiley Periodicals, Inc. (shrink)