Water is a foundational determinant of health — its availability, quality, and management directly influence disease, nutrition, and livelihoods. This book explores water supply systems, sanitation, hygiene (WASH), and their roles in preventing infectious diseases and supporting healthy communities. It blends technical guidance on treatment and distribution with behavioral strategies to improve uptake and sustainability. Risk assessment, outbreak investigation, and low-cost interventions are illustrated with community examples and monitoring approaches. Policy and governance chapters discuss equity, cross-sector collaboration, and financing for resilient water services. Suitable for health workers, engineers, and program managers working at the water–health interface.