The widespread production and use of engineered nanoparticles (ENPs) in electronics, pharmaceuticals, agriculture, and environmental remediation have raised critical concerns about their environmental fate and potential risks. Understanding the behaviour, transformation, and transport of ENPs in natural ecosystems is essential for ecological risk assessment and sustainable management. This chapter examines the key processes governing nanoparticle dynamics in environmental compartments. It highlights aggregation and agglomeration, which alter particle size and sedimentation; dissolution, which can release toxic ions; and surface modifications through adsorption of natural organic matter. Transformations such as redox reactions and sulphidation are also addressed. The role of air, water, and soil in shaping ENP behaviour is discussed with emphasis on pH, ionic strength, temperature, and natural colloids. Transport pathways, including atmospheric deposition, hydrological flow, and soil percolation, are explored alongside nanoparticle interactions with biota. Uptake by microbes, plants, and animals can lead to bioaccumulation and trophic transfer, amplifying ecological impacts. The chapter further emphasizes the significance of nanoparticle fate in exposure assessment, hazard identification, and regulatory development. Innovative tools such as computational simulations and predictive modelling are presented as vital for advancing nanoparticle-based remediation, for example, ZnO nanoparticles in wastewater treatment, while minimizing risks of toxicity and bioaccumulation.