Abstract: Marine litter is a global concern with a range of problems associated to it, as recognised by the Marine Strategy Framework Directive (MSFD). Marine litter can impact organisms at different levels of biological organization and habitats in a number of ways namely: through entanglement in, or ingestion of, litter items by individuals, resulting in death and/or severe suffering; through chemical and microbial transfer; as a vector for transport of biota and by altering or modifying assemblages of species. Marine litter is a threat not only to marine species and ecosystems but also carries a risk to human health and has significant implications to human welfare, impacting negatively vital economic sectors such as tourism, fisheries, aquaculture or energy supply and bringing economic losses to individuals, enterprises and communities. This technical report aims to provide clear insight about the major negative impacts from marine litter by describing the mechanisms of harm. Further it provides reflexions about the evidence for harm from marine litter to biota comprising the underlying aspect of animal welfare while also considering the socioeconomic effects, including the influence of marine litter on ecosystem services. General conclusions highlight that understanding the risks and uncertainties with regard to the harm caused by marine litter is closely associated with the precautionary principle. The collected evidence in this report can be regarded as a supporting step to define harm and to provide an evidence base for the various actions needed to be implemented by decision-makers. This improved knowledge about the scale of the harmful effects of marine litter will further support EU Member States (MSs) and Regional Seas Conventions (RSCs) to implement their programme of measures, regional action plans and assessments.