Exploiting wind energy at sea offers an attractive source of renewable energy avoiding problems on land, but what are the consequences for birds? We review the Danish and European experience of offshore (i.e. marine) windfarms and the effects and impacts which we consider they may have on birds, primarily through barriers to movement, displacement from ideal feeding distributions and collision mortality. We use case studies to demonstrate examples of displacement effects among species such as Red-throated Diver Gavia stellata, Common Scoter Melanitta nigra and Long-tailed Duck Clangula hyemalis but are unable to determine their causes or whether these patterns have population level impacts, assessment of which remains a major challenge. There is accumulating evidence for widespread avoidance of offshore turbines by large-bodied birds at macro-, meso- and micro-scales, but we accept that our knowledge for smaller bird species is less adequate. We conclude that careful siting during the planning phase can avoid a multitude of potential conflicts with avian populations and that despite generally inadequate post-construction monitoring (especially during periods of unusual weather), experience shows low levels of collision rates, especially among longlived large-bodied bird species considered most at risk. We lack any understanding of the impacts of barrier effects and displacement from favoured feeding areas, but on a single project basis, these impacts to date are considered insignificant at the population level because of the relatively small numbers of birds so affected. Based on experiences from multiple single site studies, it is essential that site specific impact assessments continue to be undertaken to establish the potential effects and impacts of each project development. However, we also urge a more strategic national and international approach to identification, assessment and selection process for sites for potential future development of offshore windfarms. Despite low-level impacts on an individual windfarm basis, cumulative impacts of multiple offshore windfarm development (especially spanning the length of population flyways) have yet to be adequately determined. Developing effective mechanisms to deliver such assessments remains an urgent requirement for the immediate future.