Chaperonins are ubiquitous and essential protein folding machines. They have a striking structure, with two rings of seven, eight, or nine protomers forming a "double doughnut" complex, with the cavity in each ring being the likely site for protein folding to take place. The group I chaperonins, found in bacteria and the organelles descended from them, are well characterised in terms of their structure, mechanism, and in vivo roles. The group II chaperonins, found in eukaryotic cytosol and archaea, are less well understood. In this review, we focus on what is known about the archaeal chaperonins, both in terms of their in vivo role and their structure/function relationships, in order to more fully understand their significance in archaea and as models for chaperonin function in general.