Phytopathogenic bacteria use the type III secretion system (TTSS) to inject effector proteins into plant cells. This system is essential for bacteria to multiply in plant tissue and to promote the development of disease symptoms. Until recently, little was known about the function of TTSS effectors in bacterial-plant interactions. New studies dissecting the molecular and biochemical action of TTSS effectors show that these proteins contribute to bacterial pathogenicity by interfering with plant defense signal transduction. These investigations provide us with a fresh view of how bacteria manipulate plant physiology to colonize their hosts.