Activity, morphology and size of an organelle are not constant, but vary dependent on extracellular and intracellular conditions. The endoplasmic reticulum (ER) is the location where secretory and transmembrane proteins are folded. Dysfunction or overwork of the ER, which is mostly accompanied by ER accumulation of unfolded client proteins, leads to transcriptional induction of proteins that work in and/or for the ER. This cellular event, known as the unfolded protein response (UPR), is observed in a wide variety of eukaryotic species, and its mechanism has been mainly uncovered through studies using yeast Saccharomyces cerevisiae as a simple model organism. The intracellular signaling pathway of the UPR contains various remarkable features, which include the involvement of regulatory splicing of transcription-factor mRNAs that is performed by the ER-located transmembrane endoribonuclease Ire1 in the cytoplasm. In this article, we describe our current understanding about Ire1 and the UPR in cells of S. cerevisiae and other eukaryotic species including plants.