The co-evolution of phytopathogenic bacteria and their hosts has determined the outcome of interactions between these organisms. The sophisticated plant immune system ensures the resistance of most plants to most microbial attacks and thus the ability of phytopathogens to cause disease is an exception rather than the rule in nature. In susceptible plants, however, bacterial virulence factors manipulate a number of host cellular pathways, thereby facilitating successful colonization. These virulence factors include effector proteins that are delivered into host cells via the type III secretion system (TTSS) and suppress host defenses. The type III effectors (TTEs) perturb various host cellular processes including the hypersensitive response, MAPK signaling, cellular trafficking, transcription, hormone signaling, host protein modification, and stomatal reopening. This review summarizes the observations of recent studies focusing on the interactions between the two model organisms Arabidopsis thaliana and Pseudomonas syringae that have shed light on the TTSS and the virulent activities of TTSS-translocated effector.