Xylella fastidiosa has emerged from relative obscurity into one of the most well-studied bacterial plant pathogens. While Pierce's disease of grape caused by this pathogen has been recognized as an important disease in warmer regions of the United States for nearly 100 years, the causal pathogen, X. fastidiosa has spread throughout much of the world and now also causes serious diseases of citrus, coffee, almond, olive, and other important crop plants. Our knowledge of this pathogen has been driven by the recent substantial research support justified by the economic importance of these diseases.