The 9th International Symposium on Yersinia was held in Lexington, Kentucky, USA on October 10-14, 2006. Over 250 Yersinia researchers from 18 countries gathered to present and discuss their research. In addition to 37 oral presentations, there were 150 poster presentations. This Symposium volume is based on selected presentations from the meeting and contains both reviews and research articles. It is divided into six topic areas: 1) genomics; 2) structure and metabolism; 3) regulatory mechanisms; 4) pathogenesis and host interactions; 5) molecular epidemiology and detection; and 6) vaccine and antimicrobial therapy development. Consequently, this volume covers a wide range of current research areas in the Yersinia field.