Description: The purpose of the guidelines is to review existing knowledge and provide guidance for designing an Arctic monitoring program that will track litter and MP. The topics of litter, plastic pollution, and MP are addressed in many fora, including several of the Arctic Council working groups: Arctic Monitoring and Assessment Programme (AMAP; https://www.amap.no/documents/doc/amap-assessment-2016-chemicals-of-emerging-arctic-concern/1624), Protection of the Marine Environment (PAME, 2019), and Conservation of the Arctic Flora and Fauna (CAFF). The development of an Arctic monitoring program and its technical approaches will be based on the work that already exists in other programs such as those of OSPAR, the Helsinki Commission (HELCOM), the International Council for the Exploration of the Sea (ICES), the Organisation for Economic Co-operation and Development (OECD), and the United Nations Environment Programme (UNEP). Plastic pollution is typically categorized into items and particles of macro-, micro-, and nano-sizes. These guidelines address macrosized litter as well as MP (〈 5 mm), essentially including smaller size ranges (〉1 µm). However, determination of nanoplastic (〈 1 µm) particles is still hampered by technical challenges, as addressed in Section 4.3 Analytical methods, and thus not currently considered in the current recommendations. Although most studies have addressed marine litter and MP, these guidelines also comprise the Arctic’s terrestrial and freshwater environments. Thus, the objectives of the guidelines are to: 1) support litter and MP baseline mapping in the Arctic across a wide range of environmental compartments to allow spatial and temporal comparisons in the coming years; 2) initiate monitoring to generate data to assess temporal and spatial trends; 3) recommend that Arctic countries develop and implement monitoring nationally via community-based programs and other mechanisms, in the context of a pan-Arctic program; 4) provide data that can be used with the Marine Litter Regional Action Plan (ML-RAP) to assess the effectiveness of mitigation strategies; 5) act as a catalyst for future work in the Arctic related to biological effects of plastics, including determining environmentally relevant concentrations and informing cumulative effects assessments; 6) identify areas in which research and development are needed from an Arctic perspective; and 7) provide recommendations for monitoring programs whose data will feed into future global assessments to track litter and MP in the environment. To achieve these objectives, the guidelines present indicators (with limitations) of litter and MP pollution to be applied throughout the Arctic, and thus, form the basis for circumpolar comparability of approaches and data. In addition, the guidelines present technical details for sampling, sample treatment, and plastic determination, with harmonized and potentially standardized approaches. Furthermore, recommendations are given on sampling locations and sampling frequency based on best available science to provide a sound basis for spatial and temporal trend monitoring. As new data are gathered, and appropriate power analyses can be undertaken, a review of the sampling sizes, locations, and frequencies should be initiated. Plastic pollution is a local problem in Arctic communities, and thus, guidelines and references need to include community-based monitoring projects to empower communities to establish plastics monitoring with comparable results across the Arctic. Community-based monitoring is an integrated part of the objectives of this report. The monitoring program design and guidelines for its implementation are the necessary first steps for monitoring and assessment of litter and MP in the Arctic. The work under the AMAP LMEG is taking a phased approach under this new expert group. The first phase (which included the development of these Monitoring Guidelines) focuses on a monitoring framework and set of techniques for physical plastics. Later phases of the work will extend to assessments of levels, trends, and effects of litter and MP in the Arctic environment. The guidelines strictly cover environmental monitoring of litter and MP. This does not include drinking water or indoor air quality tests. Additionally, although there is an emphasis on examining litter and MP in biota that are consumed by humans, and thus of interest to human-health questions, the guidelines do not consider MP ingestion by humans.