Turning Water Into a Tool: From Degradation Pathways to Functional Engineering in Halide Perovskites

Lead halide perovskites have emerged as promising materials for next‐generation optoelectronic devices due to their outstanding physicochemical properties and low‐cost fabrication. However, widespread application remains limited by pronounced chemical instability under ambient conditions, particularly in the presence of moisture. Traditionally viewed as a primary trigger of degradation, water promotes lattice collapse through hydration, ion migration, and photooxidative pathways. We critically analyze the multifaceted role of water in halide perovskite chemistry, shifting the paradigm from degradation toward controlled functionality. Beyond its detrimental effects, we highlight how controlled exposure to moisture can enhance material quality through water‐assisted synthesis, recrystallization, and defect healing, promoting oriented crystal growth and the formation of stable hybrid and heterostructured architectures. By integrating mechanistic insights with emerging water‐mediated processes, this review outlines new opportunities for water‐controlled synthesis and stability engineering of halide perovskites, enabling robust materials that leverage water as a functional, kinetic, and structural agent, thereby transforming a traditional challenge into a platform for innovation.