This article's goals are to: (1) investigate the current state of knowledge regarding the risks of engineered nanoparticles for the environment and human health; (2) determine whether this knowledge is sufficient to enable a thorough and efficient risk assessment of these materials; and (3) offer recommendations for future research in this area. To achieve the goals, the applicability of each of the four steps of the risk assessment methodology—hazard identification, dose-response assessment, exposure assessment, and risk characterization—was assessed in the context of the current state of knowledge regarding the risks of nanomaterials. Limitations were then noted, and suggestions were made for how to get around them.