This paper is an update of "Toxicologic Risk Assessment for Groundwater Constituents" in Water Resources Update, Autumn 1989, pp 8-13. Appropriate use and interpretation of risk assessments for groundwater that contains constituents rests on an understanding of the basic concepts of toxicology and of exposure estimation. The exposure component of risk is fraught with uncertainty. Actual constituent concentrations in the groundwater are not static over time. The assessment of toxicologic risk associated with constituents in groundwater should be undertaken with an appreciation of the science of toxicology and with respect for the many sources of uncertainty associated with both hazard and exposure.