The accumulation of protein aggregates has a fundamental role in the patophysiology of distinct neurodegenerative diseases. This phenomenon may have a common origin, where disruption of intracellular mechanisms related to protein homeostasis (here termed proteostasis) control during aging may result in abnormal protein aggregation. The unfolded protein response (UPR) embodies a major element of the proteostasis network triggered by endoplasmic reticulum (ER) stress. Chronic ER stress may operate as possible mechanism of neurodegenerative and synaptic dysfunction, and in addition contribute to the abnormal aggregation of key disease-related proteins. In this article we overview the most recent findings suggesting a causal role of ER stress in neurodegenerative diseases.