Bacterial diseases of plants are most frequent and severe in moist and warm climates. This is due, in part, to the mode of dissemination of bacteria, which primarily involves water. The spattering effect of rain, for example, enables the transmission of the pathogens from the soil, a possible source of inoculum, to plants, and from one plant to another. Bacteria penetrate inside the plant through natural openings, such as stornata or wounds, and typically multiply in the intercellular space or in the xylem (apoplast). Bacterial phytopathogens cause a wide variety of disease symptoms on many different host plants (Agrios, 1988). The best-studied bacterial phytopathogens are from the Gram-negative genera, Agrobacterium, Erwinia, Pseudomonas, Ralstonia and Xanthomonas. Some species have been further subdivided into pathovars (pv.) to distinguish strains differing in the plant species they infect, i.e. Pseudomonas syringae pv. phaseolicola causes halo blight of bean, whilst P. syringae pv. tomato is the causal agent of bacterial speck of tomato. This review focuses on the mechanisms employed by various species of Gram-negative bacteria to become successful pathogens. The bacteria are all faced with the same challenges: to suppress or avoid the host plants' defences; and to modify the local plant environment to satisfy their nutritional needs. However, the strategies employed to reach this goal differ enormously. The Gram-positive phytopathogens, Clavibacter spp. and Streptomyces spp., are not discussed in this chapter.