This paper presents an overview of the US Environmental Protection Agency's Research to Improve Health Risk Assessments (RIHRA) program and several examples of health research applicable to drinking water. Research efforts are aimed at persistently problematical issues while expanding toxicological data and analytical methodologies used in health risk assessment. Following background information on the risk assessment paradigm and the RIHRA research program, the author discusses specific areas of research: uncertainty analyses, integrated exposure assessment, physiologically based pharmacokinetic modeling, and biologically based dose-response modeling.