Abstract: Protein folding in the cell requires molecular chaperones. The chaperone proteins of the hsp70 and hsp60 (chaperonin) classes stabilize unfolded or partially folded polypeptides, thereby preventing aggregation, and mediate folding to the native state in ATP-dependent reactions. Recent advances include a more detailed understanding of the mechanistic principles of hsp70 and hsp60 action, the solution of the crystal structure of the chaperonin GroEL, and the definition of pathways of chaperone-mediated protein folding.