The Arthur D. Little concept of unit operations embodied a number of different methods of separating mixtures and represented a major advance in chemical engineering. Over time, those and subsequent concepts have evolved into a unified field of separation processes. The ways in which this happened are traced. The more unified view of separations enables more coherent and powerful approaches for process selection and design, reducing energy requirements, for selecting separating agents, understanding the complex interactions of mass transfer and phase equilibria, and identifying new methods for separating complex mixtures. As such, separation processes provide one of the most effective vehicles for teaching and understanding the engineering of chemical processes.