The Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection (GESAMP) has been involved in the issue of marine plastic litter and microplastics for over a decade. Initially interest was focussed on microplastics, which were considered as an emerging issue, and resulted in the preparation of a scoping paper. This was followed by an international workshop in 2010, organised by GESAMP, on the subject of: Microplastic particles as a vector in transporting persistent, bio-accumulating and toxic substances in the ocean. This was one of the first workshops that brought together representatives of the chemicals industry, academia, policy makers, intergovernmental organisations and NGOs. It was hosted by IOC-UNESCO in Paris, with additional financial support from the European Commission. One of the conclusions of the workshop was that further assessment of the potential impacts of microplastics was warranted. This led to the formation of GESAMP Working Group 40 (WG40) in 2012: Sources, fate and effects of microplastics in the marine environment. The first WG40 report was published in 2015: Sources, fate and effects of microplastics in the marine environment – a global assessment. The second was published in 2016: Sources, fate and effects of microplastics in the marine environment – Part two of a global assessment. It became apparent during the assessment, and preparation of the reports, that there were relatively few data available from monitoring programmes. Most data that had been published were from individual surveys or research projects, and there was a lack of harmonisation of sampling methods and attention to natural environmental variability. This made the collation and comparison of data problematic. At the same time, it was decided that the artificial cut-off imposed by only focussing on microplastics was inappropriate. Marine plastic litter covers a wide spectrum of sizes, and larger items tend to fragment to smaller particles. The title and remit of WG40 was modified to reflect this more inclusive approach. An increasing number of administrations and individual organisations have started to develop routine monitoring programmes for marine litter and microplastics, in response to greater political and social awareness. Reliable monitoring allows the setting of indicators and targets and supports decision-making. The need for greater harmonisation of methods has become more critical with the adoption of the UN Sustainable Development Goals (SDGs), in particular SDG14.1.1: floating plastic litter as a global indicator of marine pollution. This need has been recognised in resolutions passed by the UN Environment Assembly (UNEA), with GESAMP being considered an appropriate mechanism to develop appropriate recommendations. These Guidelines are the output of the third phase of WG40. It is the product of a group of dedicated independent scientists, supported by a number of national and international bodies. They are intended to provide practical guidelines and recommendations, in particular to organisations that are less experienced in marine environmental monitoring. As technologies advance, and experience is gained, the Guidelines may need to be revised. But for the moment we hope the content of this report provides a helpful contribution.