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.