This state-of-the-science review was undertaken to identify and assess currently available sampling and analysis methods to identify and quantify the occurrence of nanomaterials in the environment. The environmental and human health risks associated with nanomaterials are largely unknown, and methods needed to monitor the environmental occurrence of nanomaterials are very limited or nonexistent. Because this research is current and ongoing, much of the applicable information is found in gray literature (e.g., conference proceeding, communications with research scientists and other experts).