Quantitative neuropsychiatry has provided increasingly precise descriptions of behavioral phenotypes associated with neurodegenerative disorders. Degenerative diseases of the brain are disturbances of protein metabolism, with failure of protein degredation by the ubiquitin-proteosome system, production of neurotoxic peptide oligomers, and accumulation of intracellular protein deposits. Abnormalities of amyloid beta peptide, alpha-synuclein protein, and hyperphosphorylated tau protein account for more than 90% of degenerative dementias. Functionally related neuroanatomical systems have shared metabolic characteristics and common vulnerabilities to protein dysmetabolism, providing the basis for phenotypes that reflect the underlying proteotype. Patients with alpha-synuclein disorders are particularly prone to hallucinations, delusions, and rapid eye movement sleep behavior disorder. Patients with tauopathies manifest disproportionate disinhibition and apathy, and may exhibit compulsions. Alzheimer's disease is a triple proteinopathy with abnormalities of A-beta, tau, and alpha-synculein leading to a complex behavioral phenotype. This molecular approach to neuropsychiatry may assist in understanding the mechanisms of degenerative diseases, provide insight into the pathophysiology of neuropsychiatric symptoms, and contribute to monitoring disease-modifying therapies.