Plant pathogenic bacteria have evolved specialized strategies to infect their hosts. In this regard, the key virulence factors are effector proteins, cell wall degrading enzymes, toxins, extracellular polysaccharides and phytohormones. The interactions between plant pathogenic bacteria and their hosts have resulted in an evolutionary system between host defense responses and pathogen virulence factors. Pathogenic bacteria are continually under pressure to diversify their mechanisms to prevent host defenses and optimize nutrient availability. In turn, these virulence mechanisms have shaped the evolution of plant innate immunity. In this paper, the pathogenicity and virulence factors of plant pathogenic bacteria are discussed.