Microplastics (MPs) and nanoplastics (NPs), ubiquitous environmental pollutants resulting from the degradation of plastic waste, have emerged as significant concerns for human health. Due to their small size and high surface area- to-volume ratio, these particles can enter the human body through inhalation, ingestion, and dermal contact. Once internalized, MPs and NPs may translocate across biological barriers, accumulate in tissues, and induce toxicological responses across multiple organ systems. This review summarizes current evidence on the effects of MPs and NPs on key organ systems. Experimental and epidemiological data suggest that these particles can trigger oxidative stress, inflammation, immune dysregulation, cellular apoptosis, and even genotoxicity. However, understanding of their long-term health consequences remains limited due to variability in particle characteristics, exposure routes, and study designs.