The chapter “Ecotoxicity Assessment of Emerging Contaminants in Fish: Effects, Trends, and Challenges” examines the rising presence of emerging contaminants (ECs) in aquatic ecosystems and their toxic effects on fish populations. These contaminants, including pharmaceuticals, personal care products, and industrial chemicals, pose ecological and health risks due to their bioaccumulation and persistence in water bodies. The chapter highlights fish as sentinel species, crucial for monitoring aquatic ecosystem health, and discusses advanced ecotoxicological methods like high-throughput screening, omics technologies, and bioaccumulation studies. It also outlines the challenges in assessing ECs due to the complexity of chemical mixtures and regulatory gaps, advocating for interdisciplinary approaches to improve environmental risk assessments.