This chapter inaugurates the discussion of unit operations, by presenting the fundamental principles of distillation in binary mixtures. We start with the analysis of liquid-vapour equilibria through relative volatility, phase diagrams and equilibrium curve. These concepts provide the foundation for practical applications such as flash distillation and continuous distillation with reflux. The McCabe-Thiele’s graphical method, based on assumptions of constant molar flux and ideal equilibrium stage, is presented for the design of distillation columns. The role of operating and feed lines, reflux ratio, and energy balances in condenser and reboiler are discussed. Finally, the chapter addresses deviations from the ideal stage model by introducing the concept of Murphree’s efficiency.