Structural information on group II chaperonins became available during recent years from electron microscopy and X-ray crystallography. Three conformational states have been identified for both archaeal and eukaryotic group II chaperonins: an open state, a spherical closed conformation, and an intermediate asymmetric bullet-shaped form. However, the functional cycle of group II chaperonins appears less well understood, although major principles are conserved when compared to group I chaperonins: binding of the substrate polypeptide to the apical domains of the open state and MgATP-driven conformational changes that result in encapsulation of the substrate where folding can proceed presumably in the closed ring of the bullet-shaped form. Binding of the transition state analogue MgADP-AlF3-H2O in the crystal structure of the Thermoplasma acidophilum thermosome suggests that the closed geometry is the enzymatically active conformation that performs ATP hydrolysis. Domain movements observed by electron microscopy suggest a coupling of ATP hydrolysis and domain movement similar to that in the GroE system. The hydrophilic interior of the closed thermosome corresponds to the cis-ring of the asymmetric GroEL-GroES complex implicated in protein folding.