This paper shows that wind turbine harmonic generation and mitigation assessment are complex tasks, and many aspects, such as measurements, data processing, modelling and validation, must be taken into consideration. The paper describes the measurement process and shows sophisticated analyses on representative harmonic measurements of the wind turbine generators at Avedøre Holme. The nature of generation and mitigation of harmonic components in the wind turbine generators are clearly presented and explained. The mechanism of harmonic generation, some dynamic behaviour aspects and interaction with the external network are considered. Measurement, data processing and simulation results are presented and compared. Different analysis methods, such as statistical analysis, harmonic calculation, and mathematical description are applied and described in detail. Some issues regarding commonly applied standards are also put forward in the paper. Based on measurements and simulations, it is shown that a general overview of the behaviour of a wind turbine generator cannot be fully observed only based on harmonic current analyses as suggested in the standards.