A microemulsion, made from water, oil, surfactants and cosurfactant is a thermodynamically stable system. The presence of the cosurfactant is often required in order to lower the interfacial tension of this interface, because a low interfacial tension is essential for the formation of microemulsions. The transparency of microemulsions arises from their small droplet diameter. The droplet diameter in stable microemulsions is usually within the range of 10 - 140 nm. Microemulsions are graphically represented as stability areas in triangular phase diagrams where each triangular corner designates a certain component. Microemulsions are actually quaternary (pseudoternary) systems. In pharmaceutical fields, the interest in microemulsions is increasing and, thus, they are applied to various administration routes.