The partition of a solute between two immiscible solvents is a major technique in separation, both in the laboratory and in industry. This technique has been designated liquid–liquid distribution by the International Union of Pure and Applied Chemistry (IUPAC), but it is more commonly called solvent extraction, the term we use. Solvent extraction has been developed in a broad field of applications: (1) for studying inorganic and organic complex equilibria, either for fundamental understanding of solution equilibria and kinetics or for developing selective separation schemes; (2) for separations in analytical chemistry; (3) for large-scale industrial separation processes in the inorganic, organic, pharmaceutical, and biochemical industries; and (4) for industrial waste treatment. It also serves as a base for other analytical techniques, such as liquid partition chromatography and ion-selective electrodes.