<p>Calcium chloride (CaCl₂) is a technologically important material used in applications ranging from desiccation and thermochemical energy storage to molten-salt electrochemistry. Most notably, it is used as the electrolyte in the FFC Cambridge process for metal/alloy production. Despite its widespread use at elevated temperatures, CaCl₂ is often treated as a chemically and structurally invariant salt, with limited consideration of its solid-state history prior to melting. This review examines how hydration history, dehydration pathways, and associated microstructural evolution govern the thermodynamic stability and functional performance of CaCl₂ across solid, liquid, and electrochemical regimes. The crystal chemistry of CaCl₂ hydrates and their stepwise dehydration behavior are summarized, with emphasis on insights from in-situ high-temperature X-ray powder diffraction that reveal the coexistence of multiple hydrated, partially dehydrated, and anhydrous phases. Particular attention is given to the existence of distinct anhydrous CaCl₂ polymorphs with different unit-cell volumes, their relative stability above 150&#xa0;°C, and the role of lattice densification during dehydration. The limitations of conventional thermal analysis and the advantages of in-situ diffraction for resolving metastable intermediates and phase competition are critically discussed. Beyond the solid state, the review considers how pre-melting structural states, defects, and residual disorder may influence molten-salt structure, ionic transport, and electrochemical stability. The implications of these structure–stability relationships for molten-salt electrochemical processes are highlighted. Finally, key knowledge gaps and future research directions are identified, underscoring the need for integrated in-situ structural, thermal, and electrochemical studies to enable rational control of CaCl₂ performance in high-temperature technology applications worldwide.</p> Graphical abstract <p></p>