"Present wind turbine gearbox design usually includes one or two planetary gear stages and at least one high speed helical gear stage@ which play an important role regarding noise and vibration behavior. Next to the overall noise of the gearbox and the structure-born noise on the gearbox housing also tonal noise is becoming a much more important issue in recent years. Since tonal noise is problematic due to the human perception as ""uncomfortable""@ avoidance is important. Conventional theories regarding low noise gear design are not developed in view of tonal noise. This leads to the question: How to deal with tonal noise in the design stage and which gear parameters can be used for an optimization regarding good tonal noise behavior? Within a research project measurements have been performed on different gearboxes using different gear designs. These measurements have been evaluated according to ISO/IEC 61400-11 and the results have been analyzed in view of the influence of different gear parameters. It was also investigated if it is possible to rank gearboxes in wind turbines according to their tonal noise behavior as observed on the test rig. The paper will give an introduction into the definition of tonal noise according to ISO/IEC 61400-11 and give insight in measurement results from test rigs and from gearboxes in the field@ where noise behavior is also evaluated according to ISO/IEC 61400-11. Furthermore@ the paper will show and discuss the link between measurement results and different gear parameters@ which are affecting tonal noise behavior. In addition simulation results will be presented@ showing how tonal noise can be estimated within the design stage using state-of-the-art calculation software. The paper will give recommendations regarding a gear design process that is considering tonal noise in the design stage and will compare an@ regarding tonal noise@ improved gear set with an older one."