The intention of the series Developments in Veterinary Virology is to provide monographs dealing with the major animal viral diseases. Each volume will include the latest achievements in fundamental research and practical applications and should be readable for people from various disciplines and different backgrounds. The multi-author approach provides the best opportunity to keep each chapter at the highest level and makes the composition of the volumes manageable to the editors. This monograph on Avian Leukosis presents comprehensive reviews on the recent history of avian retrovirus research, on epizootiological, virological, pathological aspects, on tumor induction, the immune response to avian retro- viruses, virus-cell interactions and on techniques for diagnosis. The volume deals mainly with exogenous avian leukosis virus (ALV) infections, but one chapter is entirely devoted to endogenous avian leukemia virus. Molecular biology aspects are confined to various oncogenes and to lymphoma induction since retroviruses, including those specific for avian species, have recently been described in detail in the Cold Spring Harbor Laboratory series "Molecular Biology of Tumor Viruses". Two chapters are devoted to the practical application of insights obtained from avian leukosis research: influences of AL V infection on production performance and eradication procedures.