The principle purpose of this report is to provide recommendations, advice and practical guidance for establishing programmes to monitor and assess the distribution and abundance of plastic litter, also referred to as plastic debris, in the ocean. It is a product of the GESAMP Working Group (WG40) on ‘Sources, fate and effects of plastics and microplastics in the marine environment’, co-led by the Intergovernmental Commission on Oceanography (IOC-UNESCO) and the United Nations Environment Programme (UNEP). The term ‘plastic litter’ is used throughout, but is synonymous with ‘plastic debris’. In some cases the report refers to monitoring strategies and sampling protocols that have been designed for the monitoring of all forms of marine litter (i.e. processed wood, metal, textiles, glass, munitions, and plastics).