All cells depend on correctly folded proteins for optimal function. A central question in cellular biology is how such folded structures are formed and maintained, a process that is now recognized to rely heavily on a group of proteins called molecular chaperones. Molecular chaperones constitute distinct families of proteins that are ubiquitous and highly conserved from bacteria to humans. They appear to bind nonnative conformations of most, if not all, proteins, thereby preventing their aggregation and subsequent inactivation. The chaperones not only protect newly synthesized proteins during transport and folding, but also serve to maintain the cell in a healthy state during exposure to a multitude of stress conditions. Accordingly, chaperones are expressed constitutively, but their synthesis is further enhanced during stress conditions. Detailed insights into the role of molecular chaperones have come from studies of mitochondrial protein biogenesis, a process in which chaperones act as unfoldases, pulling devices, and foldases. In this review we summarize these developments and further discuss the potential role of chaperones in mitochondrial DNA metabolism and human mitochondrial disease states.