Starting from a single-stage un-cooled turbine, we extend the definition of turbine efficiency to cover both cooled and multi-stage turbines. The consideration of secondary effects such as pumping work, windage and sealing flows is discussed, and the concepts of chargeable and non-chargeable flows are introduced. It is shown that there are many options and that performance models typically use a control volume to derive an equivalent single-stage definition. The pros and cons of various approaches are discussed, and the calculated values are compared to “thermodynamic efficiency”, the ratio of actual power delivered to the cumulative power potential of each individual flow. The chapter concludes with a numerical evaluation of the efficiency loss due to cooling in a real engine.