This book records the papers and discussions at a Work­ shop which took place in London on the 5th and 6th of April 1979, as part of the programme of the Commission of the European Communities on Medical and Public Health Research. However the views expressed are those of the individuals concerned and not of the EEC or any of its organs. The object was to discuss certain biological aspects of natural and experimental slow virus infections. Because the amount of knowledge and the focus of interest varied in respect of each infection the approach and emphasis varied also. In the case of scrapie, we discussed the nature of the agent and the mode of pathogenesis, in the case of SSPE, the search for unusual features of the virus, and recent detail­ ed work on the immunology of the disease. As for Visna we reviewed the present understanding of the virus and its pathogenicity and also field epidemiology and methods for its control. There were also general papers, on interferon and oncornaviruses for example. We thank all those who made the meeting possible and enabled us to produce this book quickly, so that those who could not attend the meeting may nevertheless be able to read a great deal of what went on at it. In particular we would thank the Ciba Foundation who allowed us the use of their premises and Mrs. Jean Ashley who dealt with most of the arrangements.