Plastic Busters MPAs is a 4-year-long Interreg Med-project aiming to contribute to maintaining biodiversity and preserving natural ecosystems in pelagic and coastal Mediterranean marine protected areas (MPAs), by defining and implementing a harmonized approach against marine litter. The project entails actions that address the whole management cycle of marine litter, from monitoring and assessment to prevention and mitigation, as well as actions to strengthen networking between and among pelagic and coastal MPAs.The overarching aim of this presentation is to describe the Plastic Busters MPAs harmonized monitoring approach to detect the impact of marine litter (particularly microplastics) on Mediterranean ecosystems and marine biodiversity, including endangered species (cetaceans, sea turtles and birds) inhabiting pelagic and coastal MPAs in the largest SPAMI of the Mediterranean sea. The implementation of the monitoring strategy and the preliminary results obtained from the monitoring carried out in the Pelagic SPAMI Pelagos Sanctuary and in the Tuscan Archipelago National Park (PNAT) will be presented. In spring-summer 2019, 34 researchers of 8 European institutions, monitored more than 2230 nautical miles, collecting 140 samples of superficial microplastics in the study areas and carrying out 280 monitoring of surface macrolitter simultaneously monitoring biota. During these campaign, in order to assess the ecotoxicological impact on biodiversity, cetacean species skin biopsies, neustonic invertebrates, lantern fish, mussels and several edible fish species were collected. The entire sampling design was guided by the development of a marine litter distribution model in order to identify the possible microplastic hot spot areas and of the potential impact on biota. Furthermore, seasonal monitoring of the marine litter were regularly carried out on the beaches of the PNAT islands and the coasts of the Pelagos Sanctuary in order to identify which are the most abundant waste and the sources and then guide the mitigation actions.