Lewy bodies are distinct protein inclusions that are found in PD and are composed of aggregated synuclein. ALS is a progressive paralytic disease, involving the selective degeneration of motor neurons in the spinal ventral horn, most of the brainstem and the cerebral cortex.Many different mutations associated with familialALS lead to protein misfolding and aggregation; these mutations can affect the genes encoding superoxide dismutase and other proteins.HD is a lateonset autosomal dominant neurodegenerative disease, involving the accumulation largeprotein inclusions generated by mutant huntingtin protein owing to an expansion of a polyglutamine region.Thus, even though the clinical manifestation of all these diseases is diverse, at the molecular level they share the phenomenon of accumulation of abnormally folded proteins in the form of small oligomers, aggregates or largeprotein inclusions.This abnormal aggregation process affects the function of the endoplasmic reticulum at different levels, resulting in ER stress.ER stress may further increase aggregation of diseaserelated proteins through a feedback loop by altering the folding and quality control capacity of the cell or by modifying the expression of diseaserelated genes.ER stress engages the unfolded protein response sensors, which in turn activate distinct downstream responses.Early UPR signalling events attenuate protein synthesis at the ER by transiently inhibiting translation and by enhancing regulated inositolrequiring enzyme and autophagy.Prolonged ER stress overcomes the adaptive responses of the UPR and apoptosis is induced.CHOP also induces the expression of growth arrest and DNA damageinducible and increases protein synthesis.Altered calcium homeostasis due to inositol trisphosphate receptor may also contribute to cell death.IRE also induces the activation of JUN aminoterminal kinase, which contributes to cell death.Many correlative studies in human postmortem tissue have shown that the presence of UPR markers in the brain is temporally and spatially associated with abnormal protein aggregation and the occurrence of neuropatholog icalfeatures. Activation of proximal unfolded protein response events is also highlighted.In diseases such as ALS, the detection of ER stress markers in body fluids has even been suggested as a reliable approach to follow disease progression. From these correlative studies, the concept has emerged that ER stress is a deleterious process contributing to neurodegeneration.However, on the basis of functional studies in mouse models, it is becoming clear that the scenario is very complex, and in some diseases the engagement of specific UPR signalling events may actually operate as a beneficial reaction to maintain proteostasis.The initial prediction was that activation of the UPR contributed to neuronal loss by activation of a proapoptotic stress signal.However, more recent and extensive studies using genetic and pharmacological manipulation of key UPR components have uncovered an unexpected scenario.UPR activation can either enhance or reduce neurodegeneration and sometimes may even have opposite effects on disease progression depending on which specific UPR signalling mechanisms are activated. In this section, we summar ize the most relevant data supporting this interesting concept that is, that the consequences andor outputs of the UPR in neuro degenerative conditions can depend on the specific nature of the pathological input. ER stress was recently identified as a salient feature of neuronal cultures generated from induced pluripotent stem cells obtained from patients with PD.