The interest in mixed surfactant systems is continuously increasing. The development during the last years has mainly been focused on (i) the theoretical description of mixed micelles, (ii) the coexistence of two types of micelles, one rich in one of the surfactants and the other rich in the other surfactant, and (iii) mixtures of cationic and anionic (catanionics) surfactants forming systems in which vesicles and other intricate structures appear. This review also touches on surfactant mixtures at surfaces as well as on some examples of technical applications.