What evidence associates adverse human health effects with hazardous waste sites and other sources of uncontrolled releases of hazardous substances? How extensive and robust is the human health effects database? When should a health study be conducted? This chapter summarizes basic concepts in epidemiology and presents the key literature on associations between human exposure to hazardous substances and impacts on human health. The chapter is organized around ATSDR’s Seven Priority Health Conditions on the health effects of substances released from waste sites.