An extractant dissolved or impregnated in a porous inert support, rather than a covalently bonded ion-exchange group, constitutes the stationary phase of extraction chromatography. The mobile phase is typically an aqueous acid or salt solution; the target ion or molecule partitions into the supported organic extractant according to the same equilibria that govern liquid–liquid extraction, and is later stripped by a change of aqueous composition ^[1]. Crown ethers, organophosphorus acids, and related extractants provide selective coordination environments for strontium, actinides, and other species ^[2]. The technique is thereby a chromatographic embodiment of solvent extraction, distinct from solid-phase extraction that uses a dry sorbent without a liquid extractant film, and from reversed-phase chromatography whose retention is not based on a designed extractant–metal or extractant–solute complex ^[3]. Essential features are a defined extractant, a support that holds that extractant without dominating selectivity, and a reversible partition that can be reset by the aqueous eluent. Conceptual boundaries also exclude ordinary adsorption chromatography on bare silica or alumina.