Protein misfolding and aggregation are common to most neurodegenerative diseases, suggesting that abnormalities of protein homeostasis contribute to pathogenesis. Research implicates at least two components of cellular protein quality control in disease: molecular chaperones and the ubiquitin-proteasome pathway (UPP). Although evidence is more compelling for chaperone involvement, recent cell-based and genetic studies suggest that perturbations in the UPP also contribute to neurodegenerative disease processes. UPP involvement in disease seems even more probable when the UPP is viewed not simply as an isolated degradation machine but rather as a complex cascade linked both to other ubiquitin-dependent processes and to chaperone systems.