Nuclear technology offers a diverse set of scientific and engineering tools for addressing contemporary water challenges. Applications range from radiation-based pollutant degradation and microbial disinfection to isotope hydrology, desalination, environmental monitoring, and advanced water reuse systems. This book provides a detailed examination of the principles, technologies, operational methodologies, environmental applications, safety considerations, and future developments associated with nuclear technologies in water treatment. The integration of nuclear science with modern environmental engineering provides new opportunities for improving water quality, enhancing treatment efficiency, reducing environmental impacts, and strengthening global water security.