There are growing lines of evidence addressing a potential link between failure of protein quality control and neurodegeneration. Hence, it is important to know the quality control mechanism of the cell for understanding the molecular basis underlying neurodegenerative diseases. To date, it is notable that molecular chaperones and the ubiquitin-proteasome system play a central role for maintenance of protein homeostasis by catalyzing refolding and/or the immediate destruction of misfolded or impaired proteins generated in cells. However, how this proteolytic machinery recognizes abnormality of cellular proteins for selective elimination remains largely elusive. In this article, we discuss the mechanism of neurodegeneration, based on the protein quality control mediated by the ubiquitin-proteasome system in the cell.