This document provides the current practices, challenges and strategies for assessing risk of manufactured nanomaterials in circumstances where data is limited, and there is a necessity for more research on specific risk assessment issues. As such, the document presents an overview of the chemical risk assessment paradigm and describes how various member countries have adapted existing regulatory frameworks to the assessment of nanomaterials. It also presents the state of science on nanomaterials risk assessment and highlights priorities for research toward specific risk assessment issues.