Quantitative microbial risk assessment (QMRA) has emerged as a useful tool to develop criteria for human exposures to pathogens. There is opportunity to extend the usefulness of this tool in water and other applications if new fundamental information can be obtained to complement existing data. Such information includes effects of strain and host differences, population level disease dynamics, and ability of animal data to serve as a predictor of human potency. This paper reviews the development of QMRA and outlines the nature of additional data that would be useful for its development.