The accumulation of malfolded proteins in the endoplasmic reticulum (ER) induces ER stress, leading to the disturbance of ER function. To restore ER function and ER homeostasis, cells possess a highly specific ER quality control system termed the unfolded protein response (UPR), which increases the capacity of protein folding and reduces the amount of malfolded proteins. In case of prolonged ER stress or malfunction of the UPR, apoptosis signaling is activated. ER stress-induced apoptosis has recently been implicated in the pathogenesis of various conformational diseases. Apoptosis signal-regulating kinase 1 (ASK1), a member of the MAPK kinase kinase (MAP3K) family, is activated by ER stress and mediates apoptosis. Recent studies have shown that the ASK1 pathway is involved in ER stress-induced neuronal cell death and contributes to the pathogenesis of neurodegenerative diseases. In this review, we summarize the molecular mechanisms of the UPR and ER stress-induced apoptosis and the possible roles of ASK1 activation in neurodegenerative diseases.