Molecular chaperones comprise several highly conserved families of related proteins, many of which are also heat shock proteins. Chaperone proteins are crucial for the maintenance of native protein conformation and recent research has demonstrated several mechanisms where defective chaperone proteins have pathogenic consequences. In this article, we describe the structure and function of chaperones in bacterial and eukaryotic cells, focusing on the chaperonin class of chaperones. We then summarize contemporary research concerning the role of these proteins in several human diseases, concentrating on the genes coding for chaperone and chaperonin proteins and the importance of chaperones in neurodegenerative diseases and as modifiers of amino acid substitution mutations in other proteins.