This chapter deals with the operation of evaporation for concentrating solutions containing non-volatile solutes. After introducing the process objectives and main industrial applications, the effect of solute presence on the boiling temperature is analysed through the concept of boiling-point elevation. The design principles of the single-effect evaporator are then presented by means of mass and enthalpy balances, together with the definition of the steam economy. Finally, the discussion extends to multiple-effect systems.