This book provides a comprehensive synthesis of technology-critical elements (TCEs) as emerging metallic pollutants in aquatic environments. It examines rare earth elements, rubidium, and platinum-group metals, detailing their technological importance, extraction pathways, environmental release, and behavior in freshwater and marine systems. Emphasis is placed on speciation, bioavailability, bioaccumulation, and ecological toxicity, as well as interactions with microplastics and climate change. By integrating geochemical, ecotoxicological, and sustainability perspectives, the book highlights critical knowledge gaps, supports risk assessment, and underscores the urgent need for responsible resource management, improved regulation, long-term monitoring, and protection of aquatic ecosystems and human health.