A massive development of offshore windmill farms has been planned along theEuropean coastline.This raises important questions about the possible effects onthe marine environment.Effects during the construction period may be minimized toa negligible impact if care is taken to avoid areas containing rare habitats or species.Disturbance caused by noise,vibrations,and electromagnetic fields during windmilloperation may,with present knowledge,be considered to be of minor importance tothe marine environment.The reef effect (i.e.addition of a hard substratum),isbelieved to cause the largest impact on the marine environment and at differentscales:the micro scale,which involves material,texture,and heterogeneity of thefoundation material;the meso scale,which involves the revetments and scourprotection;and the macro scale,which encompasses the level of the entire windmillfarm.Effects on these scales are discussed in relation to results obtained fromnatural habitats,artificial reefs,and other man-made constructions at sea. A massive development of offshore windmill farms has been planned along theEuropean coastline.This raises important questions about the possible effects onthe marine environment.Effects during the construction of impact the care is taken to avoid areas containing rare habitats or species. Disturbance caused by noise, vibrations, and electromagnetic fields during windmilloperation may, with present knowledge, be considered to be minor importance tothe marine environment. The reef effect (ie, addition of a hard substratum), isbelieved to the largest impact on the marine environment and at differentscales: the micro scale, which involves material, texture, and heterogeneity of the bedding material; the meso scale, which involves the revetments and scourprotection; and the macro scale, which encompasses the level of the entire windmillfarm. Effects on these scales are discussed in relation to results obtained from natural habitats, artificial reefs, and other man -made constructions at sea.