For a number of years it has been the practice at Rolls-Royce to determine the basic aerodynamic characteristics of the turbines of our engines by tests of model turbines. The first model of the single-stage Derwent I turbine was tested In 1945 and a design point efficiency of 75 per cent was measured. Since then over 600 different turbine configurations have been tested and many of the more recent turbines have achieved efficiencies appreciably better than 90 per cent. Typical of modern turbines is the Spey two-shaft turbine in which the four stages reach a design point efficiency of a little over 92 per cent. This paper presents a description of the test techniques which are used in this work and also a simple method of correlation which relates the efficiency of some 70 turbine stages to the velocity triangles and enables the efficiency of those turbines, which fall within the scope of the correlation, to be predicted with an accuracy better than ± 2 per cent. The measured value of turbine efficiency can be markedly influenced by the measurement technique, so the way in which the turbines are manufactured and tested is described in some detail.