Quoting numbers for the efficiency of a turbine is ambiguous if it is not known how this efficiency is defined. This is especially true for a heavily cooled turbine where for the same machine the efficiency may be quoted as 88% or 91%, for example. In aero-engine industry, several different turbine efficiency bookkeeping systems are in use. Since nearly always a consortium of two or more companies is involved into any new engine project it is important to understand the various bookkeeping systems. Turbine designers and performance specialists should not quote a number for the efficiency of a cooled turbine without clearly stating on which methodology and control volume it is based. In this paper, the commonly used turbine performance bookkeeping methods are compared. The pros and cons for the different methodologies are discussed and correlations between the definitions are shown for single and multistage turbines with various amounts of cooling air. Furthermore it is pointed out, that simulating a cooled multistage turbine with an equivalent single-stage model requires the use of an equivalent Stator Outlet Temperature SOTeqv which differs from the true SOT. It depends on the bookkeeping system used what the calculated impact on turbine performance is when the amount of cooling air changes. Without carefully adhering to a unique bookkeeping system with a clearly defined control volume the probability of misunderstandings in collaborative engine development projects is not to be underestimated.