Energy is needed to separate a mixture into pure compounds. Some separation operations are highly energy consuming; others are less so. But all separations require a minimum amount of energy. Section 10.1 introduces the concept of minimum energy required for separation, independent of the process employed. This section then illustrates the minimum energy required in various separation processes such as membrane gas permeation, distillation, extraction and adsorption. How to reduce the amount of energy needed for a given separation by particular separation processes is the general topic of Section 10.2. The specific separation processes and topics covered in this section include: evaporation of water for desalination by multiple-effect evaporation, multistage flash evaporation, vapor-compression evaporation; distillation based separation of mixtures by multieffect and heat pump configuration. The role of free energy of mixing in influencing the energy requirement for separation is briefly considered here. Finally, the special considerations needed for processing dilute solutions of solutes of interest are discussed in the context of reducing the total energy required.