This assessment describes the far-reaching impacts of plastics across the planet and in our oceans. Plastics are a marker of the current geological era, the Anthropocene (Zalasiewicz et al. 2016). They have given their name to a new microbial habitat known as the plastisphere (Amaral-Zettler et al. 2020; see Glossary). Increased awareness of the negative impacts of microplastics on marine ecosystems and human health has led them to be referred to as a type of “Ocean PM2.5” akin to air pollution (i.e. particulate matter less than 2.5 micrometres [μm] in diameter) (Shu 2018). With cumulative global production of primary plastic between 1950 and 2017 estimated at 9,200 million metric tons and forecast to reach 34 billion tons by 2050 (Geyer 2020) (Figure i), the most urgent issues to be addressed are how to reduce the volume of uncontrolled or mismanaged waste streams going into the oceans (Andrades et al. 2018) and how to increase the level of recycling. Of the 7 billion tons of plastic waste generated globally so far, only an estimated 10 per cent has been recycled (Geyer 2020). The assessment sets out to address four key questions to help guide future actions: • What can new research and evidence tell us about the environmental and human health effects and impacts of marine litter, including plastics and microplastics? • What is the latest understanding of the sources, pathways, behaviour and fate of marine litter, especially plastics? • What are the most effective field, laboratory and modelling approaches for monitoring and measurement of the sources, pathways, behaviour and fate of marine litter, including all sizes of plastics? • What ongoing responses and actions, environmental technologies and business solutions exist to tackle this urgent global problem?