Streptococci and enterococci are the etiologic agents of infectious diseases that rank among the most severe in human pathology. The diagnosis, antibiotherapy, and prevention of the streptococcal diseases have improved considerably. However, the reemergence of severe streptococcal and enterococcal diseases constitutes a growing public health con cern, which remains open to scientific and medical debate. The XIII'h Lancefield International Symposium on Streptococci and Streptococcal Diseases, held at Institut Pasteur, Paris, France, September 16—2el, 1996, attracted 505 par ticipants from 43 countries. Twenty-two percent of the participants were students, a clear sign of the intense interest in this field. Of the 390 presentations made at the symposium, 260 were submitted as manuscripts for the Proceedings; we have included 249 of these in this volume. This symposium provided a forum for the presentation of the most recent findings and approaches to understanding several important fields, such as new aspects of infec tion, bacteria~host interactions, epidemiology, and molecular genetics of streptococci and enterococci. Over the last three years, the study of these subjects has expanded as increas ingly sophisticated methods of molecular analysis have been applied to investigate the bi ology of pathogenic streptococci and enterococci. Virulence, vaccine strategies, genetics, antibiotic resistance, epidemiology, and immunology are now being examined through the lens of molecular biology. The application of recently developed techniques to this field will continue to yield insight into the mechanism by which these organisms cause disease.