This chapter reviews the most relevant literature on qualitative and quantitative abnormalities in resting-state eyes-closed electroencephalographic (rsEEG) rhythms recorded in patients with dementing disorders due to Alzheimer’s disease, frontotemporal lobar degeneration, vascular disease, Parkinson’s disease, Lewy body disease, human immunodeficiency virus infection, and prion disease, mainly Creutzfeldt–Jakob disease. This condition of quiet wakefulness is the most used in clinical practice, as it involves a simple, innocuous, quick, noninvasive, and cost-effective procedure that can be repeated many times without effects of stress, learning, or habituation. While rsEEG has a limited diagnostic value (not reflecting peculiar pathophysiological processes directly), delta, theta, and alpha rhythms might be promising candidates as “topographical markers” for the prognosis and monitoring of disease evolution and therapy response, at least for the most diffuse dementing disorders. More research is needed before those topographical biomarkers can be proposed for routine clinical applications.