A comprehensive understanding of the biology of prion diseases is crucial for risk assessment as regards both humans and animals. To further the communication of the current knowledge and the exchange of diagnostic technologies in the scientific community, a symposium on the 'Characterization and Diagnosis of Prion Diseases' was held from September 23-25, 1999 in Tübingen, Germany. The overwhelming interest and the participation of more than 500 scientists from 26 countries made this symposium the largest meeting ever held in the field.In this Supplementum to Archives of Virology twenty-six invited speakers comprehensively present their data on the pathogenesis of prion diseases in humans and animals, on molecular mechanisms involved in the transmissibility across species barriers, on animal and in-vitro models currently available for the detection and quantification of infectivity and on the characterization of prion strains. This selection attempts to reflect the current state of the art, but cannot possibly represent the whole spectrum of research in the prion field.