This method is for the determination of microplastics (State Water Resources Control Board, 2020) greater than 20 µm in size in treated drinking water using visual microscopy for particle counts, and infrared spectroscopy for chemical identification of counted particles. This method is for use in the California EPA’s data gathering and monitoring programs and Section 116376 of the California Health and Safety Code. The method is based on peer-reviewed literature and the results and recommendations from an international microplastic method evaluation study carried out by the Southern California Coastal Water Research Project Authority (SCCWRP).