Plant pathogenic bacteria differ from nonpathogenic relatives in being able to cause diseases (physiological damage) on susceptible plants (hosts). Following infection, which in nature usually starts with low numbers of pathogen cells (propagules), they colonize and multiply profusely in living plant tissues, attaining very high population levels per tissue mass/area (typically up to several million-fold over initial inoculum concentrations). Owing to the large amounts of microbial biomass in the infected tissue and the production/release of bioactive compounds that interfere directly with biochemical signaling pathways and host physiology and gene regulation, there is excessive drain of host nutrients, interference with local and long-distance nutrient transport, and alteration in the plant’s developmental program. The infected plant displays macroscopic alterations (disease symptoms) that are fairly characteristic of the pathogen/host combination. The ultimate consequence of infection is reduced plant growth and yield, lower product quality, postharvest deterioration of crop product, destruction of plantations of perennial crops, and, in certain cases, increased susceptibility to other biotic or abiotic factors (e.g., frosts). Yield losses due to bacterial diseases as a whole may be less than the losses caused by other plant pathogens such as viruses or fungi, but can be substantial locally, regionally, or worldwide.