This book reviews understanding of the biological roles of extracellular molecular chaperones. It provides an overview of the structure and function of molecular chaperones, their role in the cellular response to stress and their disposition within the cell. It also questions the basic paradigm of molecular chaperone biology - that these proteins are first and foremost protein-folding molecules. Paradigms of protein secretion are reviewed and the evolving concept of proteins (such as molecular chaperones) as multi-functional molecules for which the term 'moonlighting proteins' has been introduced is discussed. The role of exogenous molecular chaperones as cell regulators is examined and the physiological and pathophysiological role that molecular chaperones play is described. In the final section, the potential therapeutic use of molecular chaperones is described and the final chapter asks the question - what does the future hold for the extracellular biology of molecular chaperones?