Abstract
<jats:p>Water security has emerged as a critical and multidimensional challenge of the twenty-first century, encompassing biophysical limits, socio-economic dynamics, governance systems, technological resilience, and geopolitical stability. With the global population projected to rise from 7.7 billion to nearly 11.2 billion by 2100, ensuring adequate, safe, and equitable access to freshwater will become increasingly complex. Freshwater resources are fundamental not only for drinking and sanitation but also for sustaining agriculture, ecosystems, energy production, industry, and economic development. Climate change is intensifying blue-water variability and hydrological extremes, while agricultural expansion is altering green-water fluxes and increasing pressure on land–water systems. Strategically important headwater regions, though vital for downstream water supply, remain ecologically fragile and insufficiently protected. Simultaneously, urban and peri-urban areas face growing risks from infrastructure stress, contamination, groundwater depletion, and emerging cyber-physical threats, despite the potential of decentralized solutions such as rainwater harvesting and distributed storage. Water insecurity now reflects both quantitative scarcity and qualitative degradation, with severity varying across regions, particularly in arid and semi-arid zones. This chapter advances an integrated scientific and political understanding of water security, highlighting its multi-scalar, cross-sectoral nature and examining diverse regional experiences to underscore the urgency of resilient and equitable water governance.</jats:p>