Background Important Bird Areas (IBAs) are sites that are essential to ensure the long-term viability of bird populations, particularly of those species that are of conservation concern. BirdLife International’s IBA inventories are used, amongst others, to analyse whether the current SPA network is adequate or not. Whereas the terrestrial breeding colonies of marine birds are generally well protected, the definition of marine SPAs at sea has represented a gap in the overall Natura 2000 network. Levels of quantitative data on bird populations and their distribution often decrease with increasing distance from shore. Objectives The project aimed to prepare a detailed inventory, using objective methodological criteria, to determine marine IBAs for seabird species listed in Annex I of the Birds Directive that live in Spanish marine waters. The project worked closely in co-ordination with a similar LIFE project covering Portuguese territorial waters. Results The project’s main objectives were achieved and resulted in the definition of marine IBAs in Spain. Furthermore, outcomes from the project work provide a model methodology for defining future marine IBAs, and the approach has now been adopted by members of BirdLife International as part of a global standard. This has been applied in countries such as Greece, Malta, the Baltic Countries, Argentina, Peru, USA, South Africa and New Zealand. The process started with an initial pilot methodology for defining marine IBAs that was prepared on the basis of expert advice during an international seminar organised by the project. This first prototype model was then further fine-tuned during consultation with the Portuguese counterpart project (LIFE 04 NAT/P/0213 IBA Marinha) and representatives from the Birds Habitats Directive Task Force of BirdLife International. Scientific data formed the heart of the methodology and LIFE support was vital for testing the most effective means of collating the necessary information. This involved coordinating a wide range of different data sources from a variety of different stakeholders. The results led to scientific information being secured from: bibliographic and satellite sources on sea parameters; scientific institutions proving data concerning long term seabird population trends; analysis of data provided from records of stranded seabirds and recovered rings; monitoring bird displacement trends using different tracking devices (PTT, GPS-loggers and radio-tracking); and carrying out field censuses within Spanish waters using strip-transects on boats. All this information was then converted into an inventory for testing the methodology. Findings proved so valuable, in terms of identifying sensitive marine areas, that the Spanish government decided to apply the project’s interim result during their decision making process for locating off-shore wind farms. Final details for the methodology were then agreed following additional consultations with national and international partners. Overall outputs from the final methodology have been impressive and led to the identification of 42 marine IBA, which encompass 42 883 km2 (about 5% of the Spanish marine waters). These IBAs provide habitats for some 27 different sea bird species, including 16 species from Annex I of the Birds Directive. Three different types of IBA have been classified by the methodology: ‘At-sea congregations’, ‘Migratory bottleneck areas’ and ‘Foraging Areas Important for Pelagic Birds’. Four additional areas (covering 25 000 km2) of importance to sea bird conservation have also been identified outside the Spanish jurisdictional waters and these highlight a need for wide international cooperation. Key achievements from the project come in the commitments made by national authorities to mainstream the LIFE project methodology findings by proposing some of the new IBAs to the European Commission. Moreover the project’s inventory will be used during the forthcoming designation of marine SPAs in Spain. LIFE is expected to provide support during this process as a means of adding value to the successes of this project’s good practice nature conservation outcomes.