The functioning and features of the new software package VAT (Virulence Analysis Tool) are introduced. VAT provides a range of methods for the analysis of plant pathosystems. The techniques are applicable to other binary data sets that are organized in large two-way tables, e.g., molecular marker data. The main features are data entry, descriptive tools, and inference statistics by resampling. About 50 well-established or newly developed indices allow a detailed diversity analysis of sexually and asexually reproducing populations. VAT facilitates a comprehensive, effective, and logically consistent evaluation and presentation of virulence and resistance data. A translation option simplifies the comparison of results from differently coded pathotypes. The software package comes with a detailed manual and is freely available on the internet at tau.ac.il/lifesci/departments/plant_s/members/kosman/VAT.html and at va-tipp.de .