The US Environmental Protection Agency (EPA) Integrated Risk Information System (IRIS) program develops toxicologic assessments of environmental contaminants. IRIS assessments provide hazard identification and dose-response assessment information. The information is then used in conjunction with exposure information to characterize risks to public health and may be used in risk-based decisionmaking, in regulatory actions, and for other risk-management purposes. Since the middle 1990s, EPA has been in the process of updating the IRIS assessment of inorganic arsenic. In response to a congressional mandate for an independent review of the IRIS assessment of inorganic arsenic, EPA requested that the National Research Council convene a committee to conduct a two-phase study. Critical Aspects of EPA's IRIS Assessment of Inorganic Arsenic is the report of the first phase of that study. This report evaluates critical scientific issues in assessing cancer and noncancer effects of oral exposure to inorganic arsenic and offers recommendations on how the issues could be addressed in EPA's IRIS assessment.