Cells respond to the accumulation of unfolded proteins by activating signal transduction cascades that improve protein folding. One example of such a cascade is the unfolded protein response (UPR), which senses protein folding stress in the endoplasmic reticulum (ER) and leads to improvement in the protein folding and processing capacity of the organelle. A central paradox of the UPR, and indeed of all such stress pathways, is that the response is designed to facilitate both adaptation to stress and apoptosis, depending upon the nature and severity of the stressor. Understanding how the UPR can allow for adaptation, instead of apoptosis, is of tremendous physiological importance. Recent advances have improved our understanding of ER stress and the vertebrate UPR, which suggest possible mechanisms by which cells adapt to chronic stress.