What began as a molecular curiosity in fruit flies over 30 years ago (Ritossa

1962), the so-called heat shock or stress response, now constitutes an active area of research in cell biology and biochemistry. The heat shock proteins (hsps), one of the most highly conserved group of proteins so far charac terized, are being implicated as essential components in a number of diverse biological processes. Although referred to as heat shock proteins, most of these proteins in fact are expressed at rather significant levels in all cells maintained under normal growth conditions and are essential for cellular growth at all physiologically relevant temperatures. Much of the current interest in the hsps follows from recent studies demonstrating their role as molecular" chaperones", being intimately involved in various steps of protein maturation. Members of the Hsp70 (DnaK) and Hsp60 (GroEL) families, for example, participate in protein folding, protein translocation, and perhaps higher ordered protein assembly. Yet other members of the heat shock protein family, such as Hsp90, play important roles in the regulation of certain transcription factors and protein kinases. Here, we present our current understanding of the structure and function of those hsps that function as molecular chaperones and discuss their roles both within the normal cell, as well as in the cell experiencing metabolic stress. In addition, we discuss other possible roles of molecular chaperones as they relate to various macromolecular assembly/disassembly events that accompany a number of other important biologial processes in the cell. For more details of the biochemistry, the reader is referred to a recent review on protein folding