This chapter describes new approaches using the plant Arabidopsis thaliana (Arabidopsis) as a model host. The development of Arabidopsis as a model system for plant molecular biology sparked a search for pathogens of Arabidopsis. Such pathogens were identified either by identifying naturally occurring infections on Arabidopsis, or by screening pathogens collected from other plants, especially pathogens of other crucifers. Many plant-pathogen interactions are characterized genetically by a "gene-for-gene" relationship. In such interactions, resistance of a plant to a given pathogen requires a specific R-gene in the plant and a matching dominant avirulence gene in the pathogen. Arabidopsis has been embraced by molecular biologists as the plant of choice for studying most aspects of plant biology. The primary reason for choosing Arabidopsis is that it is particularly well suited to position-cloning approaches. It is a true diploid and can be readily outcrossed or selfed. Its generation time is less than 6 weeks and individual plants can produce thousands of seed.