Chapter 4 examines current knowledge concerning Alzheimer’s disease and Parkinson’s disease with regard to prevalence, sex, race and ethnicity, and comorbidity findings. The chapter delves into etiologic theories of both disorders, including the role of genetics and environmental factors. Information regarding twin studies, early and late onset, risk, and protective factors, as well as results of structural, molecular, and functional studies is presented. The chapter concludes with research findings concerning pharmacological and non-pharmacological treatment of Alzheimer’s and Parkinson’s diseases, along with information concerning augmentation and experimental interventions.