The endoplasmic reticulum (ER) is the principal organelle responsible for several specific cellular functions including synthesis and folding of secretory or membrane proteins,lipid metabolism,and Ca2+ storage.Different physiological as well as pathological stress conditions can,however,perturb ER homeostasis,giving rise to an accumulation of unfolded or misfolded proteins in the ER lumen,a condition termed ER stress.To deal with an increased folding demand,cells activate the unfolded protein response (UPR),which is initially protective but can become detrimental if ER stress is severe and prolonged.Accumulating evidence demonstrates a link between the UPR and ovarian development and function,including follicular growth and maturation,follicular atresia,and corpus luteum biogenesis.Additionally,ER stress and the UPR may also play an important role in the ovary under pathological conditions.Understanding the molecular mechanisms related to the dual role of unfolded protein response in the ovarian physiology and pathology may reveal the pathogenesis of some reproductive endocrine diseases and provide a new guidance to improve the assisted reproductive technology.Here we review the current literature and discuss concepts and progress in understanding the UPR,and we also analyze the role of ER stress and the UPR in the ovary.