Abstract: The unfolded protein response (UPR) is an adaptive stress response in which cells recover from the accumulation of unfolded or misfolded proteins in the endoplasmic reticulum (ER) by increasing its protein-folding capacity. The IRE1 pathway in the UPR is evolutionarily conserved from yeast to human, and two other pathways involving PERK and ATF6 have also evolved in higher eukaryotes. These three intracellular signaling pathways originate in the ER lumen, where unfolded or misfolded proteins are recognized by the three transmembrane ER stress sensors IRE1, PERK, and ATF6. This review focuses on current progress with efforts to elucidate how stress sensors recognize the accumulation of unfolded proteins.