Climate change is a major global challenge, dramatically affecting water resource, hydrological systems and undermining water security worldwide. Water, an essential natural resource, is highly susceptible to changes in certain factors such as temperature, precipitation, and atmospheric circulation, making it an important indicator of climate impacts. The chapter explores the complex interactions between climate variability and hydrological cycles, showing how climate-induced warming (greenhouse gases) alters precipitation patterns, evaporation rates, and snow and glacier dynamics. Applying global climate models coupled with hydrological forecasts, the analysis addresses changes to ground and surface water resources. Vital topics include shifts in water availability, such as evolving precipitation intensity and seasonality, glacier retreat in regions such as Asia and South America, and variability in rainfall across sub-Saharan Africa. Water quality is also significantly impacted, with a rising temperature fueling algal blooms, salinization of coaster aquifers, increased sediment loads from extreme rainfall, and emerging pollutants posing escalating threats. The chapter further evaluates hydrological extremes including droughts floods, cyclones, and river flow variability, and their cascading impacts on water urban supplies agriculture, public health, energy generation, and natural ecosystems. Discussions focus on vulnerabilities in regions including Africa, Asia, Europe, the Americas, and Small Island Developing State. The chapter also reviews mitigation and adaptation strategies aimed at reinforcing resilience and facilitating sustainable water resource management.