Dementia is a clinical syndrome associated with declines in memory and other cognitive domains that are sufficient to impair daily function. The characteristic disease course of neurodegenerative dementia is an insidious onset that gradually progresses. A growing body of research indicates that both genetically predisposed and cognitively normal individuals may go through an extended asymptomatic phase, termed the preclinical stage of dementia. At this stage, toxic proteins gradually accumulate in the patient’s brain and can be detected using molecular imaging or biofluid assays in vivo. Previous studies have demonstrated that these toxic proteins begin to accumulate decades before cognitive decline occurs. In this chapter, we focus on the pathology, genetics, biomarkers, and clinical characteristics of dementia from the preclinical stage.