The endoplasmic reticulum (ER) is an intracellular organelle consisting of a membranous labyrinth network that extends throughout the cytoplasm of the cell and is contiguous with the nuclear envelope. In all eukaryotic cells, the ER is the site where folding and assembly occurs for proteins destined to the extracellular space, plasma membrane, and the exo/endocytic compartments. The ER is exquisitely sensitive to alterations in homeostasis, and provides stringent quality control systems to ensure that only correctly folded proteins transit to the Golgi and unfolded or misfolded proteins are retained and ultimately degraded. A number of biochemical and physiologic stimuli, such as perturbation in calcium homeostasis or redox status, elevated secretory protein synthesis, expression of misfolded proteins, sugar/glucose deprivation, altered glycosylation, and overloading of cholesterol can disrupt ER homeostasis, impose stress to the ER, and subsequently lead to accumulation of unfolded or misfolded proteins in the ER lumen. The ER has evolved highly specific signaling pathways called the unfolded protein response (UPR) to cope with the accumulation of unfolded or misfolded proteins. Recent discoveries of the mechanisms of ER stress signaling have led to major new insights into the diverse cellular and physiologic processes that are regulated by the UPR. This review summarizes the complex regulation of UPR signaling and its relevance to human physiology and disease.