Errors associated with the measurements of the wind speed are the major sources of uncertainties in power performance testing of wind turbines. Field comparisons of well-calibrated anemometers show a significant and not acceptable difference. TheEuropean CLASSCUP research project posed the objectives to quantify the errors associated with the use of cup anemometers, and to determine the requirements for an optimum design of a cup anemometer, and to develop a classification system forquantification of systematic errors of cup anemometers. The present report describes this proposed classification system. A classification method for cup anemometers has been developed, which proposes general external operational ranges to be used. Anormal category range connected to ideal sites of the IEC power performance standard was made, and another extended category range for complex terrain was proposed. General classification indices were proposed for all types of cup anemometers. As a resultof the classification, the cup anemometer will be assigned to a certain class: 0.5, 1, 2, 3 or 5 with corresponding intrinsic errors (%) as a vector instrument (3D) or as a horizontal instrument (2D). The classification of three commercial cup anemometersshowed that for the normal category, the best class for horizontal wind speed measurements was class 2 and for vector measurements class 3. The CLASSCUP prototype anemometer got a class 2 as a horizontal anemometer and class 1 as a vector anemometer. Forthe extended category the commercial cup anemometers were class 5 both for horizontal and vector measurements, whereas the CLASSCUP anemometer got a class 3 as a vector instrument.