This policy brief, titled "Plastics Fate and Effects in the Human Body," presents findings from the PlasticsFatE project, funded under the European Union's Horizon 2020 Research and Innovation programme (Grant Agreement No. 965367). The brief addresses the pressing need for harmonization and standardization in micro- and nanoplastics (MNPs) exposure and hazard assessment methodologies. Spanning April 2021 to March 2025, the PlasticsFatE project aims to bridge knowledge gaps regarding the effects of MNPs and associated additives or contaminants on human health. It highlights how varying methodologies and the lack of standardized approaches limit the comparability of studies, affecting regulatory decision-making. Key recommendations focus on: Developing standardized methods for sampling, preparation, and testing across multiple media. Ensuring quality assurance and interlaboratory comparisons to improve measurement reliability. Promoting harmonized toxicity testing reflecting realistic human exposure scenarios. The brief emphasizes the global challenge posed by MNP pollution, its pervasive nature across the life cycle of plastics, and its potential health and environmental impacts. Enhanced harmonization and standardization are advocated as essential steps toward more reliable, actionable data to inform policy and regulatory frameworks. DOI: 10.5281/zenodo.13861226 Keywords: Microplastics, Nanoplastics, Human Health, Environmental Impact, Standardization, Harmonization, Policy Recommendations, Policy Brief, PlasticsFatE, Horizon 2020.