Every commercial formulation that contains surfactant contains a mixture of surfactants. This chapter is a short review on surfactant mixtures in water focusing on important concepts that differ from single-component systems. The chapter assumes a basic understanding of surfactants. Topics include micelles; adsorption at the air–water, oil–water, and water–solid interfaces; and phase behavior/precipitation. Mixtures of surfactant types that are important in applications are emphasized. In addition, those molecular aspects that affect macroscopic behavior, especially macroscopic behavior that is important for applications of surfactants, are emphasized.