Emerging contaminants (ECs), encompassing pharmaceuticals, personal care products, endocrine-disrupting chemicals, microplastics, and nanomaterials, have garnered increasing global attention due to their pervasive presence and potential ecological and human health impacts. Unlike conventional pollutants, ECs are not yet comprehensively regulated, and their behaviour in various environmental matrices remains insufficiently understood. This chapter provides a critical evaluation of the fate and transformation pathways of ECs across diverse ecosystems, including aquatic (freshwater, marine), terrestrial (agricultural, forested), and extreme environments (polar, desert, and geothermal systems). Emphasis is placed on abiotic and biotic degradation mechanisms, bioaccumulation potential, and transport dynamics, with cross-comparative analysis among ecosystems to highlight differential vulnerabilities and contaminant behaviour. Furthermore, the chapter synthesizes empirical findings through case summaries that elucidate real-world trajectories of ECs under varying ecological and geochemical conditions. Special attention is devoted to anthropogenic influences, climate change, and ecosystem-specific microbial activities that modulate contaminant persistence and toxicity. Finally, the chapter outlines current challenges, data gaps, and methodological constraints, and recommends integrative monitoring frameworks and policy approaches to inform sustainable environmental governance. This chapter aims to serve as a foundational reference for future research and regulatory advancements in the field of emerging contaminant science.