The phase diagram approach to microemulsions was introduced decades ago by Gillberg and collaborators [1]. At that time, it was not well received by the researchers in the area, because it emphasized that microemulsions are in fact micellar systems and the traditionally simplifi ed thermodynamic treatment was very much in vogue at that time. Unfortunately, the ensuing arguments about the “true structure of microemulsions” shrouded the advantage of the approach, and it was only after the Israelachvili-Ninham analysis of the thermodynamics of such systems [2] that attention could be directed to the essential features of the phase diagram approach. A brief history of the development has been given by Lindman and Friberg [3].