This chapter provides an overview of delirium and dementia, including primary dementias, secondary dementias, and reversible dementias. The distinction between mild and major neurocognitive disorders is described. An in-depth description of Alzheimer’s disease, including etiology, treatment, and general guidelines for care, is included. Other dementias including Huntington’s disease, frontotemporal lobar degeneration, Lewy body disease, Creutzfeldt-Jakob disease, and vascular dementia are also briefly covered. Secondary dementias (e.g., dementia associated with Parkinson’s disease) and reversible dementias (e.g., normal pressure hydrocephalus) conclude the chapter.