In the near future the areal extent of the planned offshore wind farms (OWF) will be round about 25 % of the German exclusive economic zone (Status: Jan. 2012). The potential OWF areas are mostly characterised by considerable water depths (30 - 40 m) and distances from the coast (40 - 190 m). Each OWF project requires an environmental impact assessment (EIA) based on a BACI sampling design (before-after-control-impact). The soft bottom benthos as well as the benthic communities of the artificial hard structures has to be surveyed by various sampling techniques. Next to the EIA a research project, studying the effects of the OWF alpha ventus (German Bight, North Sea) on the endobenthic assemblages, is applied. First results showed that differences in the endobenthic assemblages among the impact and control locations significantly increased from 2008, before alpha ventus was built, to the 1st year of operation in 2010. For example, after the construction the abundance pattern of some species changed in the alpha ventus area relative to the control locations. Moreover, from before to after the construction mean species richness vary significantly in the alpha ventus area, but not in the control locations. If the detected changes are related to the wind turbines or are due to the exclusion of trawling should reveal by further studies.