Loneliness has been linked to an increased risk of cognitive impairment and dementia. This chapter reviews the literature on the relationship between loneliness and various cognitive outcomes across the dementia spectrum—from early cognitive complaints and decline before diagnosis to the accelerated progression of symptoms following disease onset. It also underscores the need for further research to identify at-risk populations based on sex, age, and other sociodemographic factors, as well as to elucidate the specific biological, behavioral, and psychological mechanisms underlying the association between loneliness and dementia. A deeper understanding of the nature of loneliness (e.g., persistent vs. transient, emotional vs. social), along with its context and sources, is essential to guide the development and implementation of interventions aimed at promoting social integration and, in turn, supporting health and cognitive function in aging adults.