Risk assessments are often focused on a single chemical. People, however, are exposed to mixtures of chemicals over their lifetime, and many argue that a better way to estimate risk is to assess exposure to mixtures, particularly for mixtures of chemicals that have similar mechanisms of toxicity or that produce similar effects. Because phthalates make up a chemical class that produce similar effects and have similar chemical structures, the U.S. Environmental Protection Agency (EPA) asked the National Research Council (NRC) to evaluate their health risks and determine whether a cumulative risk assessment would be appropriate and, if so, suggest an approach to such an assessment. In this report, the Committee on the Health Risks of Phthalates reviews risk-assessment practices and describes their strengths and weaknesses. The committee reviews the toxicity of and exposure to phthalates, considers the value of conducting a cumulative risk assessment of this chemical class, and provides recommendations for conducting the assessment.