Perspective on water-induced colloidal CsPbBr3 metal halide perovskites and their non-perovskite derivatives.

All-inorganic CsPbBr3 metal halide perovskite nanocrystals have attracted significant attention for their outstanding optoelectronic properties, yet their pronounced sensitivity to water has long been considered a critical limitation. Here, we provide a unifying perspective that reframes water not merely as a degradation factor, but also as an adaptable chemical control that promotes functional diversification in colloidal Cs-Pb-Br systems. We discuss the mechanisms underlying water-induced transformations and the formation of key non-perovskite derivatives that are water-resistant, such as PbBrOH, CsPb2Br5, and Cs4PbBr6, and highlight their structural relationships with the parent perovskite lattice. Emphasis is placed on their emerging functionalities as wide-bandgap hosts, surface passivation layers, and heterostructure components, as well as on their dynamic, often reversible interconversion with CsPbBr3 under controlled conditions. We further discuss how to enhance stability and enable tunable optoelectronic performance across applications with the water-resistant non-perovskite derivatives, including photodetectors, light-emitting diodes, photovoltaics, and bioimaging. Finally, we outline key challenges and future directions toward predictive control of moisture-driven phase behaviour, emphasising the need for in situ characterisation and mechanistic understanding. This perspective establishes water as a powerful synthetic and functional tool, offering new design paradigms for next-generation perovskite nanomaterials.