The purpose of this report is to conduct a review of the existing methodologies which assess plastic waste flows and leakage into the environment and more specifically, plastic leakage into waterways and entering oceans and seas. Although there are various methods and tools to assess plastic pollution, it is difficult amongst others to measure how much plastic is entering the oceans and thus to compare this. By developing a comparative framework, the reader can find support in identifying the methodology which best enables monitoring projects or national programs to be assessed within their context of application and in terms of their added value for preventing plastic pollution and marine litter.