The interest in mixed surfactant systems is continuously increasing. The development during past years has mainly been focused on the theoretical description of mixed micelles, the coexistence of two types of micelle, one rich in one of the surfactants and the other rich in the other surfactant, and mixtures of cationic and anionic (catanionics) surfactants forming systems in which vesicles and other intricate structures appear. There have also been advances in the understanding of surfactant mixtures at surfaces as well as technical applications.