The article discusses the monitoring of plastic debris in marine environments, emphasizing the environmental and economic impacts of plastic pollution. It highlights the challenges in assessing the efficacy of measures to reduce plastic abundance due to spatial and temporal heterogeneity and limited understanding of plastic pathways and long-term fate. The paper reviews various monitoring approaches, including beach surveys, at-sea sampling, and monitoring impacts on wildlife, and provides guidelines for best practices. Key points include: 1. **Beach Surveys**: These are cost-effective but suffer from biases due to differential removal by beachcombing, cleanups, and beach dynamics. Standing-stock surveys can provide gross changes in abundance but are less sensitive to changes in macro-litter abundance. Accumulation studies, which track the rate of new debris arriving on beaches, are more reliable but require more resources. 2. **At-Sea Surveys**: These are more challenging and expensive but provide a more accurate picture of plastic abundance at sea. Direct observations and net trawls are used to estimate floating and suspended debris, while trawl surveys assess macro-debris on the seabed. Stratified random sampling and neuston nets with a 0.33 mm mesh are recommended for effective monitoring. 3. **Monitoring Impacts on Wildlife**: Entanglement and ingestion are major impacts. Entanglement rates can indicate changes in debris abundance, but they are rare and require constant search effort. Ingestion rates in seabirds, especially petrels and storm-petrels, are useful indicators of changes in plastic debris at sea. 4. **Monitoring Sources**: Assessing the magnitude of debris sources is crucial. Ship-based sources can be monitored through independent observers and port reception facilities, while land-based sources can be sampled from rivers, stormwater runoff, and other key sources. The article emphasizes the need for standardized monitoring protocols and the importance of combining different approaches to answer specific questions about plastic pollution. It also highlights the role of education and legislation in reducing plastic waste.The article discusses the monitoring of plastic debris in marine environments, emphasizing the environmental and economic impacts of plastic pollution. It highlights the challenges in assessing the efficacy of measures to reduce plastic abundance due to spatial and temporal heterogeneity and limited understanding of plastic pathways and long-term fate. The paper reviews various monitoring approaches, including beach surveys, at-sea sampling, and monitoring impacts on wildlife, and provides guidelines for best practices. Key points include: 1. **Beach Surveys**: These are cost-effective but suffer from biases due to differential removal by beachcombing, cleanups, and beach dynamics. Standing-stock surveys can provide gross changes in abundance but are less sensitive to changes in macro-litter abundance. Accumulation studies, which track the rate of new debris arriving on beaches, are more reliable but require more resources. 2. **At-Sea Surveys**: These are more challenging and expensive but provide a more accurate picture of plastic abundance at sea. Direct observations and net trawls are used to estimate floating and suspended debris, while trawl surveys assess macro-debris on the seabed. Stratified random sampling and neuston nets with a 0.33 mm mesh are recommended for effective monitoring. 3. **Monitoring Impacts on Wildlife**: Entanglement and ingestion are major impacts. Entanglement rates can indicate changes in debris abundance, but they are rare and require constant search effort. Ingestion rates in seabirds, especially petrels and storm-petrels, are useful indicators of changes in plastic debris at sea. 4. **Monitoring Sources**: Assessing the magnitude of debris sources is crucial. Ship-based sources can be monitored through independent observers and port reception facilities, while land-based sources can be sampled from rivers, stormwater runoff, and other key sources. The article emphasizes the need for standardized monitoring protocols and the importance of combining different approaches to answer specific questions about plastic pollution. It also highlights the role of education and legislation in reducing plastic waste.