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The phase behavior of mixtures of water, oils, nonionic amphiphiles, and electrolytes (microemulsions) follows general patterns that originate from the interplay between the lower miscibility gap of the binary mixture oil-amphiphile and the upper (closed) miscibility gap of the binary mixture water-amphiphile (section II). As a consequence, the critical line, connecting the plait points of the isothermal phase diagrams of the ternary mixtures, changes from the oil-rich to the water-rich side with rising temperature. If the chemical potentials of the components are appropriately changed, the critical line may be “broken” at a tricritical point, which gives rise tothe evolution of a three-phase body (III). In the amphiphile-rich phase of the three-phase body (the microemulsion), one finds—for thermodynamic reasons—a maximum of the mutual solubility between water and oil combined with a minimum of the interfacial tension …