The Q7 is the second offshore wind farm in the Dutch sector of the North Sea and, at 23 km off the Dutch coast, the world's first to be located outside the 12-mile limit. To support the wind turbines, monopiles, 54 metre long steel pipes with a diameter of 4 metres, are hammered into the seabed. The underwater radiated noise during the impulsive hammering of 9 out of 61 monopiles was measured. Although there is a wide concern about the impact of piling noise on marine life, there are no widely accepted criteria for the maximum acceptable noise levels. A quantitative comparison of the results of carious reported studies is difficult, due to the lack of standardization in the level definitions and data processing. The Q7 data have been analyzed in terms of a broadband sound exposure level, peak pressure and pulse duration and a 1/3-octave band frequency spectrum of the sound exposure at different hydrophone locations, for each hammer stroke that has been recorded. The results are discussed in relation to the stroke energy, the hydrophone distance and depth and to proposed noise criteria for marine mammals.