Between 1999 and 2005 harbour seals and grey seals were studied to evaluate their use of Horns Rev and Nysted wind farms and the surrounding areas. Further the effect of construction and operation on resting behaviour on land and population development of seals in the general area was investigated. Both wind farm areas were part of much larger foraging areas. No general change in behaviour at sea or on land could be linked to the wind farms. The only effect detected on land was a reduction in seals resting on land during pile drivings at Nysted Offshore Wind Farm. Ship surveys observations at Horns Rev Offshore Wind Farm supports this observation, as no seals were observed in the wind farm during pile driving.