This report provides an update and further assessment of the sources, fate and effects of microplastics in the marine environment, carried out by Working Group 40 (WG40) of GESAMP (The Joint Group of Experts on Scientific Aspects of Marine Protection). It follows publication of the first assessment report in this series in April 2015 (GESAMP 2015). The issue of marine plastic litter was raised during the inaugural meeting of the United Nations Environment Assembly (UNEA) in June 2014. Delegates from 160 countries adopted Resolution 1/6 on ‘Marine plastic debris and microplastics’ (Annex I). The resolution welcomed the work being undertaken by GESAMP on microplastics and requested the Executive Director of UNEP to carry out a study on marine plastics and microplastics. This was to be based on a combination of existing and new studies, including WG40. This provided the motivation for GESAMP to revise the original terms of reference to reflect both the request from UNEP to contribute to the UNEA study, and the key recommendations from the WG40 2015 report. Each main section begins with key messages followed by a short summary of related findings from the first report. Each section ends with conclusions, knowledge gaps and research priorities. Greater effort has been made to describe the nature, distribution and magnitude of sources of macro- and microplastics. These are described by sea-based and land-based sectors, together with the main entry points to the ocean. Spatial (regional) and temporal differences in both sources and entry points are examined. One previously unrecognized source of secondary microplastics highlighted is debris from vehicle tyres. The distribution of microplastics in the five main ocean compartments (sea surface, water column, shoreline, seabed and biota) are described, together with the transport mechanisms that regulate fluxes between compartments. Regional ‘hot-spots’ of sources, distribution and accumulation zones are reported, in response to the UNEA request. The effects of microplastics on marine biota have been explored in greater detail. Greater attention has been given to the interaction of microplastics with biota. A comprehensive literature review has been assembled with tables summarising the occurrence of microplastics in a wide variety of marine organisms and seabirds. There does appear to be an association between uptake of microplastics and changes in the physiological or biochemical response in some species, observed in laboratory experiments. It is not clear whether this will be significant at a population level with current observed microplastic numbers. The current understanding of the interaction of plasticassociated chemicals with biota is reviewed, using laboratory-based experiments, theoretical studies and field-based observations. It appears very likely that this interaction will be dependent on: i) the species; ii) the relative degree of contamination of the plastic, the biota concerned and the marine environment (sediment, water, foodstuff) in that region; iii) the size, shape and type of plastics; and iv) several time-related variables (e.g. environmental transport, gut transfer, absorption/ desorption rates). This remains a contentious area of research. The occurrence of nano-sized plastics in the marine environment has yet to be established and we are dependent on drawing inferences from other fields of science and medicine when considering possible effects. Microplastics can act as vectors for both indigenous and non-indigenous species. Examples include pathogenic Vibrio bacteria, eggs of marine insects and the resting stages of several jellyfish species. A new section considers the possible effect of microplastics on commercial fish and shellfish. Microplastics have been found in a variety of commercial fish and shellfish, including samples purchased from retail outlets. Generally the numbers of particles per organism are very small, even for filter-feeding bivalves in coastal areas bordered by high coastal populations. At these levels it is not considered likely that microplastics will influence the breeding/development success of fish stocks (food security) nor represent an objective risk to human health (food safety). However, data are rather scarce and this is an area that justifies further attention. The economic aspects of microplastic contamination are considered in another new section. This relies heavily on studies looking at the effects of macrodebris on various sectors (e.g. fisheries, shipping, tourism, waste management), given the paucity of knowledge of direct economic effects of microplastics. Acting on macroplastics may be easier to justify, as the social, ecological and economic effects are easier to demonstrate. This in turn will reduce the quantities of secondary microplastics being generated in the ocean. One significant cost that may be incurred would be the provision of wastewater treatment capable of filtering out microplastics. Such systems are relatively common in some rich countries but absent in many developing nations. Clearly, there are many other reasons to introduce improved wastewater treatment (nutrient reduction, disease prevention), with reduction in microplastics being an additional benefit. Social aspects are focused around factors influencing long-term behaviour change, including risk perceptions, perceived responsibility and the influence of demographics. This is key to implementing effective, acceptable measures. A separate section summarizes good practice guidance on sampling and analysis at sea, in sediments and in biological samples. There are no global ‘standards’ but if these guidelines are followed then it will be easier to generate quality-assured data, in a cost-effective manner, and for datasets to be compared and combined with more confidence. The final main section presents an initial risk assessment framework. Having described some basic principles about risk, likelihood and consequences the section provides a conceptual framework and two case examples (one real, one hypothetical) of how the framework can be utilized. The report concludes with key conclusions and recommendations for further research.