This article reviews caregiver burden studies that evaluated burden of care for a mentally ill relative using measurement instruments with established validity and reliability. The review identifies aspects of caregiving that are most burdensome to caregivers. It describes the nature of the relationships between variables and different dimensions of caregiver burden, and identifies mixed findings that are theoretically relevant to caregiver burden. The review discusses research findings in light of the methodological issues and research designs characterizing the literature, and briefly summarizes the effects of burden on the caregiver's life. Finally, it identifies advances made in this line of research in recent years and highlights areas that need further attention in future research work. Summary tables are included.