Bacterial pathogens employ type IV secretion systems (T4SSs) for various purposes to aid in survival and proliferation in eukaryotic hosts. One large T4SS subfamily, the conjugation systems, confers a selective advantage to the invading pathogen in clinical settings through dissemination of antibiotic resistance genes and virulence traits. Besides their intrinsic importance as principle contributors to the emergence of multiply drug-resistant "superbugs," detailed studies of these highly tractable systems have generated important new insights into the mode of action and architectures of paradigmatic T4SSs as a foundation for future efforts aimed at suppressing T4SS machine function. Over the past decade, extensive work on the second large T4SS subfamily, the effector translocators, has identified a myriad of mechanisms employed by pathogens to subvert, subdue, or bypass cellular processes and signaling pathways of the host cell. An overarching theme in the evolution of many effectors is that of molecular mimicry. These effectors carry domains similar to those of eukaryotic proteins and exert their effects through stealthy interdigitation of cellular pathways, often with the outcome not of inducing irreversible cell damage but rather of reversibly modulating cellular functions. This article summarizes the major developments for the actively studied pathogens with an emphasis on the structural and functional diversity of the T4SSs and the emerging common themes surrounding effector function in the human host.