Molecular chaperones are a set of conserved protein families that share the remarkable ability to recognize and selectively bind nonnative proteins under physiological and stress conditions. Thus, they prevent irreversible aggregation reactions and keep proteins on the productive folding pathway. Evidence suggests that the cell has developed several functionally distinct chaperone families to support protein folding. The importance of molecular chaperones under stress conditions is highlighted by the finding that all the major heat shock protein families (Hsp104, Hsp90, Hsp70, Hsp60/GroEL, and small Hsps) suppress irreversible unfolding reactions. Under heat shock, only the increased expression of a repertoire of different chaperones seems to convey thermotolerance and guarantee survival. The molecular mechanism by which chaperones in general influence protein folding processes is still far from clear. However, significant progress has been achieved in understanding some of the partial reactions of the chaperone folding cycles and in functionally differentiating between the different chaperone families.