Biological function of proteins largely depends on their conformation, therefore the folding process by which proteins arrange in a proper three-dimensional structure is extremely important and controlled in the cell. Folding is assisted by molecular chaperones that interact with newly synthesized proteins. The endoplasmic reticulum (ER) is an important organelle in which almost a third of cellular proteins are folded and modified. If ER homeostasis is disturbed, unfolded proteins accumulate in its lumen leading to a condition known as endoplasmic reticulum stress. As a result, a cellular defense mechanism known as the unfolded protein response (UPR) is activated. UPR strives to reduce the content of unfolded proteins in the ER. Signal transduction is induced by sensory proteins located in the ER membrane. Ire1 is present in yeast, and proteins IRE1, PERK and ATF6 are present in Metazoans. The result of the signaling cascade is increased transcription of genes whose products are involved in protein folding and ER membrane synthesis, and also reduction of global protein synthesis. If protein cannot be properly folded, it is degraded by the process called endoplasmic reticulum-associated protein degradation (ERAD) which is also regulated by the UPR. Proper functioning of these mechanisms is important for the organism since the accumulation of unfolded proteins and formation of protein aggregates can cause various diseases.