The endoplasmic reticulum (ER) is responsible for proper folding of secretory and membrane-bound proteins as well as the degradation of improperly folded proteins. Fluctuations in protein folding demand exceeding folding capacity result from events such as cellular stress and mutations affecting protein folding. Detrimental accumulation of unfolded proteins within the ER is alleviated through activation of evolutionarily conserved, intracellular signaling proteins by the unfolded protein response (UPR). Binding of membrane-bound signaling proteins by inactive, ER-resident chaperones typically results in suppression of UPR. However, upon nascent protein accumulation, chaperone recruitment allows for activation of stress relieving pathways. UPR, classically studied in budding yeast and later in metazoan and plant cells, relies almost exclusively on the signal proteins, known as Ire1p. Homologs for the metazoan UPR sensors include Ire1α and Ire1β as well as two additional signal proteins, PERK and ATF6. Plant UPR branches identified to date include IRE1a, IRE1b, bZIP17 and bZIP28. In this review, we present the first comprehensive view of both conserved and differing aspects of UPR across kingdoms, with special emphasis on some unique features of recently discovered plant UPR pathways.