The European wind industry uses wind turbine analysis codes for the calculation of dynamic loads and energy yield. The codes are based on detailed aeroelastic and structural models. A stochastic wind simulator is part of the code: It provides the turbulent wind over the rotor plane, using statistical properties of the wind (i.e. mean wind speeds, turbulence intensities, turbulent length scales etc) as a basis. The results of the design codes (i.e. loads, power, control variables etc. as function of time) are important for the design of wind turbine (components) and for certification purposes. In Europe different codes are used which are developed by several organisations. In the past several projects have been performed, which aimed at the determination of the accuracy and reliability of such codes. However, usually the number of codes which were involved in these projects was limited and the attention was often focussed on particular sub-models of the design codes, i.e. the modelling and verification of dynamic stall effects, dynamic inflow effects etc. Consequently, a general insight on the accuracy and reliability of the present most widely used codes is lacking in the industry and certification institutes. The objectives of the project were thus defined as: The assessment of the accuracy and reliability of the most widely used European wind turbine design codes for improved support of wind turbine design and certification; The definition of recommendations for improvement of the present wind turbine design codes and the required supporting experiments. To this end an overall verification of the most widely used European wind turbine codes are performed. Eight wind turbine codes from five different countries were involved. Code predictions (mainly loads: blade loads, rotor loads and tower loads but also accelerations and inflow velocities) have been compared with measurements, which are obtained at different conditions (normal operating conditions and special events). The experimental data are collected on three different turbines: The Nordtank-500 (NTK-500): This is a three bladed stall controlled, 41 m diameter constant speed turbine. The turbine is located at RISØ's test field near Roskilde Denmark and the measurements are performed by RISØ; The Tacke-500 turbine: This is a three bladed stall controlled, 37 m diameter constant speed turbine. The turbine is located on Crete, Greece in complex terrain. The measurements are performed by CRES; The Lagerwey 750 turbine (LW750): This is a three bladed, active pitch, variable speed, and direct drive turbine with a diameter of 50.5 meter. This turbine is located in Oude Tonge (The Netherlands) and the measurements are performed by ECN. This project is a combined research and development project, containing the verification and application of aero elastic design tools to wind turbine engineering. The main results are the insights into the accuracy and reliability of the major European design codes, and a database, which contains a comparison between calculated and measured loads. The insights from the project will be used by the participants to improve the quality of the design support they offer to the industry. The importance of the database lies in the fact that design codes are regularly updated. The quality improvement of updated codes will be assessed by comparing the results from the updated codes with the results from the database.