Microplastics (MPs) are ubiquitous contaminants in the marine environment, posing a significant threat to marine organisms and ecosystems. This chapter delves into the molecular pathways underlying the fate and effects of MPs in the oceans, providing a comprehensive overview for researchers, policymakers, and environmentalists. We first explore the sources and distribution of MPs, outlining the pathways by which these particles enter the marine ecosystem. We then examine the interactions between marine organisms and MPs, focusing on ingestion, absorption, and accumulation within different species. The chapter further investigates the molecular responses of marine organisms to MP exposure, examining the activation of stress response pathways, including oxidative stress and inflammation, and potential disruptions to cellular processes like gene expression and cell signaling. We also analyze the transfer of MPs and associated contaminants through the food chain, highlighting the molecular mechanisms driving bioaccumulation and biomagnification in higher trophic levels. Finally, we discuss the potential long-term implications of MP exposure on marine ecosystems, emphasizing the need for further research to better understand the complex molecular pathways involved. The chapter concludes by underscoring the urgency of developing effective mitigation strategies to minimize the impacts of MPs on the marine environment and preserve the invaluable ecosystem services it provides.