This paper provides a review of research issues and findings on the epidemiology of delirium. Despite the fact that research on this important geriatric syndrome has been conducted for many decades, several methodological issues make it difficult to compare findings across studies. In this paper we first discuss broadly methodological issues related to diagnosis, case-finding, and populations studied. We next review data on the occurrence and consequences of the syndrome. A discussion of the design and preliminary results of the Commonwealth-Harvard Study of Delirium in Elderly Hospitalized Patients documents both how we responded to the methodological issues outlined and how these choices influenced our findings. We conclude with a discussion of the needs for further research on the epidemiology of delirium.