Inositol-requiring enzyme 1 (IRE1) is a bifunctional serine/threonine kinase and endoribonuclease that is a major mediator of the unfolded protein response (UPR) during endoplasmic reticulum (ER) stress. It represents a potential therapeutic target for a number of diseases associated with endoplasmic reticulum stress. IRE1 is the only identified ER stress sensor in yeast and essential for UPR in animals and plants. As an ER transmembrane protein, IRE1 monitors ER homeostasis through an ER luminal stress-sensing domain and triggers UPR through a cytoplasmic kinase domain and an RNase domain. Upon ER stress, IRE1 RNase is activated through conformational change, autophosphorylation, and higher order oligomerization. Mammalian IRE1 initiates diverse downstream signaling of the UPR either through unconventional splicing of the transcription factor Xbp-1 or and through posttranscriptional modifications via Regulated IRE1-Dependent Decay (RIDD) of multiple substrates.