A review of modern stoichiometric ion exchange theories of solvent extraction separations is presented. These theories are named here as conventional or classic theories. The chapter provides an insight into the basic theories and chemistry of the process. This includes: - the physical and chemical principles and formal expressions of the process (basic solution mechanisms and thermodynamics); - a survey of the effects on the distribution between immiscible [0]phases; - chemistry and other aspects of the extractants, diluents and modifiers, and their interactions in the solvent extraction process; - aspects of the dispersion, and coalescence of phases, changes in parameter values, such as concentration of solutes and solvents, complex formers and other reactants, pH, temperature, etc.