Arabidopsis thaliana (Arabidopsis) is proving to be an ideal model system for studies of host defense responses to pathogen attack. The Arabidopsis genetic system is significantly more tractable than those of other plant species, and Arabidopsis exhibits all of the major kinds of defense responses described in other plants. A large number of virulent and avirulent bacterial, fungal, and viral pathogens of Arabidopsis have been collected. In the last few years, a large number of mutations have been identified in Arabidopsis that cause a wide variety of specific defense-related phenotypes. Analysis of these mutant phenotypes is beginning to give glimpses into the complex signal transduction pathways leading to the induction of the defense responses involved in protecting plants from pathogen infection.