The purpose of this thesis is to examine the issue of noise from wind turbines and eventually optimize their operation settings. The tools used are both simulations and measurements. The programming part was based on an existing model by Wei Jun Zhu developed in DTU as part of another master thesis. The code was expanded, combined with a BEM code and coupled to an optimization routine. All simulations and tests were made on a SIEMENS SWT-2.3-92 wind turbine equipped with a B45 blade. Noise measurements were taken at the Risø test site for large wind turbines at Høvsøre. A total of eleven hours of data were obtained in two days and they featured measuring noise versus different pitch angles and rotational velocities. The first aim of this project was to validate the code predictions against the measurements, in a way that it had not been possible to do before. In addition, to make a detailed study of the individual noise mechanisms along the blade and with changing wind speed. Subsequently, the code was going to be used for optimizing the performance of the SWT-2.3-92 wind turbine with respect to noise. This is a variable speed, pitch regulated machine, so the project concentrated on looking for the combinations of these settings that would lead to a possible reduction in noise by keeping the power production at high levels. Alternatively, to find the settings that optimize power production by constraining the maximum allowed noise. The ultimate ambition was to see whether the design of the blade itself could be modified for it to become more silent. This study concentrated on the chord, twist and relative thickness distributions.