Book excerpt: The strict relationships between bacteria and plants represents one of the major facets of terrestrial ecology. Depending on the type of interaction and amount of metabolic advantage one organism can obtain from such relationships, these are classified as mutualistic, commensal or parasitic interactions. Within this context, Pseudomonas and Xanthomonas are bacterial genera with a worldwide spread, capable of establishing all of the above mentioned interactions with plants. Therefore, they represent good models for studying different lifestyles and, accordingly, deciphering distinct evolutionary trajectories followed by different lineages of a single genus to infect and/or to establish a mutualistic relationships with the plant. Some members of these two genera are regulated pests that are recognized as economically major threats for their host crop(s) both in temperate and tropical environments.