The stress response is ubiquitous among all organisms of the bacterial, plant, and animal kingdoms. A prominent feature of this response is the synthesis of a discrete set of proteins, known as heat shock proteins, that have been shown to be essential in a plethora of protein biosynthetic and processing reactions, including protein folding and oligomerization, translocation, and secretion. During heat shock and other forms of physiological stress, heat shock proteins act as intracellular sentinels to recognize malfolded proteins. The heat shock response is tightly regulated and encompasses selective transcriptional and translational mechanisms that control the preferential synthesis of stress proteins during physiological stress. Deregulation of stress gene expression is associated with various human diseases. It is likely that the balance of interactions between damaged protein molecules and stress proteins has profound effects that impinge on normal cell growth and differentiation.