Molecular chaperones are cellular machines that facilitate protein folding. The crystal structures of HtpG, the Escherichia coli homolog of hsp90, reported in this issue (Shiau et al., 2006) together with the recently published structures of an hsp90-cochaperone complex (Ali et al., 2006) and an hsp90-client protein complex (Vaughan et al., 2006), reveal exciting insights into the hsp90 reaction cycle.