Among all anthropogenic materials ever manufactured, plastic is by far the most versatile, with a wide range of applications, from product packaging to medical equipment. Its worldwide exponential use alongside inefficient waste management have led to the accumulation of large and small plastic items in the environment. Once released in the environment, plastics undergoes weathering and biofouling processes that might contribute to the fragmentation of larger items into microplastics. MicroplastiX is a JPI Oceans international interdisciplinary research project aiming to improve the understanding of the degradation mechanisms that affect microplastics in the ocean. To overcome knowledge gaps on weathering, degradation, and fragmentation, MicroplastiX will implement a holistic approach that combines field data with laboratory experiments, while evaluating the interactions of plastic items with biota in the water column, both horizontally and vertically. Several case studies in different environmental matrices will be explored to assess abundance and distribution pathways in relation to depth and distance to the shore. Concerning fate and impacts of MPs in marine organisms, the intake, identification, quantification of MPs and plastic-associated chemicals in marine biota (e.g. fish and mollusks) will be assessed in several ecosystems, ecological habits, and temporal scales.The project will also explore biofilm growth and colonization rates through ad-hoc in-situ and laboratory experiments, to assess how MP buoyancy, environmental dynamics, and transported contaminants affect microorganisms growing on the plastic surface. The planned surveys, laboratory activities and analytical development will estimate MP ingestion and egestion in zooplankton and other model organisms. Based on the data collected, a set of multiscale numerical models will be developed to predict the fate and pathways of microplastics in the environment. This will contribute to better decision-making and help to identify potential critical matrices of plastic accumulation.