Deployment of offshore wind farms (OWF) is rapidly expanding these years. A Before-After-ControlImpact (BACI) approach was used to study the impact of one of the world’s largest off shore wind farms (Horns Rev Offshore Wind Farm) on fish assemblages and species diversity. Fish were generally more abundant in the Control than the Impact area before the establishment of the OWF. Eight years later fish abundance was similar in both the Impact and Control area but the abundance of one of the most frequently occurring species, whiting, was much lower as compared to 2001. However, the changes in whiting reflected the general trend of the whiting population in the North Sea. The introduction of hard bottom resulted in higher species diversity close to each turbine with a clear spatial (horizontal) distribution. New reef fishes such as goldsinny wrasse, Ctenolabrus rupestris, viviparous eelpout, Zoarces viviparous, and lumpsucker, Cyclopterus lumpus, established themselves on the introduced reef area. In contrast very few gobies were caught near or at the OWF, presumably owing to the highly turbulent hydrographical conditions in the OWF. We suggest that the lack of this common prey fish is the main reason for the absence of larger predatory fish species