The 9th International Symposium on Yersinia was held in Lexington, Kentucky, USA, in October 2006. Hundreds of 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 that include genomics and pathogenesis.