Anthropogenic litter is found in marine environments from the beach, beach sediment and surface water to the seafloor. Plastic can persist and accumulate in the marine environment for a long period because of its light weight and degradation-resistant properties. The global production of plastic increased from 230 million tonnes in 2005 to 322 million tonnes in 2015. It has been estimated that plastic production will increase to 330 million tonnes in 2017. Mismanaged plastic waste can enter the marine environment via both land-based sources and oceanic-based sources. Ocean gyres, oceanic convergence zones and even polar regions are regarded as plastic waste accumulation hotspots. Over 690 species including seabirds, turtles, and fish have been reported to ingest plastic debris. Additionally, large plastic debris, especially derelict fishing gear and packing bags, pose a plastic entanglement risk to marine organisms. Plastic debris can act as vectors for the accumulation of hydrophobic organic pollutants and heavy metals or metalloids, and thus potentially cause harmful effects to marine organisms, such as endocrine system disruption, liver and kidney failure, hormone alterations and teratogenicity. This chapter summarizes the sources, distributions and fates of plastic debris in the marine environment, as well as the physical and chemical effects induced by plastic debris. Finally, recommendations including legislation reinforcement, better management of waste collection systems and landfills, raising the awareness of different stakeholders and the application of advanced technology are suggested to reduce the amount of plastics in the marine environment.