The objectives of this study were (1) to develop methodology for characterization and detection of AuNPs and AgNPs size distribution and particle number concentration in environmental water, (2) to track the stability of AuNPs and AgNPs at environmentally relevant con- centrations, and (3) to analyze AuNPs and AgNPs in environmental samples. In this study, we chose AuNP and AgNP as the representative metallic nanoparticle because of their broad applications and potential release into the environment.