The purpose of this report is to provide guidelines for the assessment of plastic contamination, from macro- to microplastics, in freshwater environments. It provides the most current procedures for monitoring and analysing plastic content in rivers, lakes, reservoirs and water/wastewater treatment plants. Recommendations have been developed reflecting stakeholder inputs from a series of workshops, which revealed that developing and developed countries face similar challenges with the implementation of monitoring programmes for plastics in freshwater environments. However, the type and intensity of hurdles to be overcome in setting up monitoring programmes may differ in different countries. The guidelines are designed to assist in the timely development and implementation of freshwater plastic monitoring programmes, tailored to the different starting conditions in the different countries.