This "Book of Abstracts" is the main publication of the 55th Annual Meeting of the European Association for Animal Production (EAAP) held in Bled, Slovenia, on 5-9 September 2004. It contains abstracts of the invited papers and contributed presentations. The meeting addressed subjects relating to science and innovation, but in Bled the main theme is "Sustainability of Livestock Systems". Also, important problems were discussed during the sessions of EAAP"s nine Commissions: Animal Genetics, Animal Nutrition, Animal Management and Health, Animal Physiology, Livestock Farming Systems, Cattle Production, Sheep and Goat Production, Pig Production and Horse Production. In addition joint sessions on topics interesting several disciplines and species were included in the programme. A total of 2843 authors (741 abstracts) contributed to this publication.