Wind farms have a negative impact on several animal populations, with birds and bats assumed to be among the most heavily affected taxa. In addition to the extensive development of wind power on land, currently several European countries have offshore wind farms (OWFs) in operation, as well as proposals to build many more. Considering that seabirds spend more than 90% of their life at sea, it is clear that they may interact with these man-made structures. To date, information on how to measure the impact of OWFs on seabirds is very scarce. In many cases it comes from non-peer reviewed reports made for specific assessment studies, very often carried out without appropriate standard methodologies, that raise questions regarding the level of confidence that they merit. In this paper, we describe the current political framework concerning the possible development of OWFs in Spain. Based on this, we comment on both the relevant variables necessary to assess the impacts of OWFs on birds and the methodologies available to collect those variables in the field, discussing the strengths and weaknesses of each method. We show the importance of collision risk models, vulnerability indexes, cumulative impact assessment and population viability analysis as management tools. Mitigation measures that have the potential to minimise impacts are also summarised. We conclude with a discussion of the different steps to follow to attain a 'clean' development of offshore wind energy in Spain.