This chapter presents the fundamentals of the other stage unit operation chosen to open this book, i.e. liquid-liquid extraction, focusing on the transfer of a single solute from a carrier solvent into an extracting one under the assumption of ideal (equilibrium) stages. The discussion begins with graphical tools for representing ternary mixtures, with particular emphasis on triangular diagrams to solve mass balances. The principles of liquid-liquid equilibria are then presented, highlighting binodal curves, tie lines, distribution coefficients, and selectivity, all of which guide the choice of a suitable solvent. Attention then turns to the single-stage extraction process, where overall and component balances are treated graphically via the Hunter-Nash’s method, and the operating limits of solvent flow rate are identified. The treatment is extended to multistage arrangements, including crossflow and countercurrent schemes. For each configuration, design and verification strategies are outlined, showing how multiple stages increase recovery efficiency, while overcoming the inherent constraints of single-contact operations.