This master thesis was carried out within SP Technical Research Institute in Borås, under the instruction of Professor Per Fahlén and Dr. Roger Nordman. It focuses on studying different air-to-air heat pump COP (Coefficient of Performance) field testing methods and standards. Practical measurements with three test methods were carried out in parallel in the laboratory, namely SP Method, Climacheck Method and Calorimeter Method. Results were collected from both long and short term testing with the corresponding outdoor weather conditions. Hence, heat pump performance characteristics can be studied. Furthermore, uncertainty evaluation and limitation analysis of each testing method were made so that further improvement could be suggested for field measurement. While defrosting is a key factor to ensure high efficiency, the dynamic performance within that period is extremely hard to monitor, and it is critical for the long term HSPF (Heating Seasonal Performance Factor) measurement of air-to-air heat pumps.