The number of offshore wind farms (OWFs) is increasing rapidly, leading to questions about the cumulative environmental impact of such farms. The major impacts are caused by the noise produced in the building phase, the new hard substratum, the moving rotor blades, possible underwater noise in the production phase and the exclusion of fisheries. Danish, Dutch, British, German and Belgium monitoring programmes in individual wind farms have shown, in general, an increase in biodiversity due to the hard substratum, as well as changes in benthic and fish fauna due to the presence of the foundations and further away due to exclusion of fisheries and attraction or avoidance by some bird species or sea mammals. These effects can be qualified as partly negative, partly positive and partly unknown or so far unclear. In particular, the cumulative effects of the increasing number of wind farms need further attention. Negative effects of wind farms include effects of noise on sea mammals and possibly fish, avoidance behaviour by some bird species, and bird and bat collisions with turning rotor blades. If OWFs want to be real suppliers of green energy, ways to mitigate the negative effects need further research and development of innovative techniques.