Of the 8,300 million tonnes of plastic produced from 1950 to 2015, only 7% has been recycled while more than half has been discarded in landfill or leaked into the environment. Companies, organisations, and governments are taking measures to tackle plastic pollution. However, there is currently no standard methodology to measure the extent of the plastic problem. This report provides a review of existing and emerging methodologies to identify the abundance, distribution, types, sources, pathways and sinks of plastic pollution at different scales. It also provides an overview of the state of knowledge for impact assessment and monetary valuation methodologies, along with a glossary of key terms related to plastics, marine plastics and environmental footprints.