Heat shock proteins (Hsps) are molecular chaperones that oppose stress-induced denaturation of other proteins. Hsps are present in all organisms. Apart from assisting in the efficient folding of newly synthesized proteins they maintain pre-existing proteins in a stable conformation, preventing their aggregation, under stress conditions. The latter role, essential for thermal adaptation, requires that the chaperone system change from a folding to a storing function at heat shock temperatures. The temperature at which this change occurs depends on the presence of a thermosensor in at least one of the components of the chaperone systems. In this review, we focus on the bacterial GroE and DnaK systems, describe their temperature-sensitive protein components, and the location of the thermosensor within the structure of these components. While the thermosensor of the GroE system is located at the inter-ring interface of GroEL, that of the DnaK system occurs in its co-chaperone GrpE. Analysis of these examples demonstrates the amazing mechanistic diversity of thermal stress adaptation and of functional convergence of structurally unrelated proteins.