In this review article, we describe the historical background of gerontopsychiatry in Japan and elucidate research studies on dementia. To date, 19 relatively large-scale epidemiological investigations on dementia have been conducted in the community. These results suggest that approximately 4–5% of the population over age 65 has dementia. Multi-infarct dementia is more prevalent than the Alzheimer's type dementia. Three hypotheses for this finding are suggested. Finally, we propose a theoretical model correlating the relationship between degree of cognitive dysfunction and caregiver's stress.