The European Seabirds at Sea (ESAS) database was established in the early 1980s using a common format. It contains the results of ship-based and aerial seabird surveys in the Northwest European waters, collected using standard methods. The emphasis has always been on mapping distribution patterns and variations in relative abundance, from which seabird abundance estimates for certain sea areas could be made. The data have been used first to evaluate different sea areas in terms of their vulnerability for surface pollutants. Later studies put more emphasis on ecological aspects underlying seabird distribution and from this work a growing need for more adequate, but still standardised, coding of behaviour types emerged. In this manual, a coding system is introduced that allows specific coding of associations of birds and marine mammals with certain surface phenomena (including land), coding of multi-species feeding associations (feeding flocks) and coding of a variety of behaviour types, with emphasis on feeding behaviour and foraging interactions. In this coding method, the original aim and style of data collecting of seabirds at sea remains intact, and new data are therefore directly comparable with historical material. The coding is thought to be of interest for ESAS participants as well as other groups studying the behaviour of seabirds at sea.