This paper explores the adequacy of using currently available exposure assessment models for indoor air inhalation exposures in the context of the uncertainty that exists in both the dose-response assessment and the exposure assessment. A tiered system is proposed for implementing exposure assessments. Each tier involves additional research, but also would lead to reductions in uncertainty. The authors discuss a possible comprehensive research program that will permit the building of well-validated models for indoor air inhalation exposures among classes of chemicals, building types, and exposure scenarios. The authors believe that until such a research program is undertaken, modeling of exposures (and therefore risks) using the current, unvalidated modeling approaches provides little more than worst-case estimates that are useful primarily for making screening decisions about risks.