California Senate Bill 1422 requires the State Water Resources Control Board to adopt a definition of microplastics in drinking water, develop a standard methodology to measure microplastics in drinking water, monitor for microplastics in drinking water for four years and publicly disclose results, and consider a health-based guidance level to aid consumer interpretations of the results of the testing, as well as accredit laboratories to analyze microplastics. California state scientists are collaborating with academic researchers, laboratories, instrument manufacturers (including Thermo Fisher), and industry to advance the science and complete these ambitious objectives. This presentation will provide an overview of the world’s first regulatory investigation of microplastics in drinking water, including the definition of microplastics, known human health effects, method standardization efforts, and upcoming plans and actions regarding microplastics in drinking water.