Ion exchange is one of the most versatile separation technologies in water treatment, hydrometallurgy, catalysis, pharmaceuticals, and food processing. Its materials fall into two broad families: synthetic exchangers, dominated by cross-linked polymeric resins, and natural exchangers such as zeolites, clays, and biopolymers. This review examines how these materials differ in three performance dimensions that govern practical use: selectivity, exchange capacity, and regenerability. It first summarises the structural and thermodynamic foundations of ion exchange, then compares conventional resins, chelating and hybrid exchangers, inorganic synthetic materials, and natural exchangers. Recent advances in nanocomposite sorbents, ion-imprinted polymers, and low-chemical regeneration are discussed alongside persistent challenges, including fouling, regenerant waste, and selectivity for emerging contaminants. It also presents the governing equilibrium, kinetic and fixed-bed equations, characterisation methods, industrial case studies, and the economics of regeneration. The review closes with research priorities for sustainable, circular ion exchange processes.