Bacterial invasion has been a very active area of research due to the extensive amount of morbidity and mortality that can be attributed to this group of pathogens. Analysis of many of these pathogens is hampered because they are either obligate intracellular pathogens or because genetic tools to dissect the process have not been available. Nevertheless, a number of invasive bacteria have been studied in considerable detail, and the knowledge gained from these studies serves as a model for future research. Invasion (and intracellular survival where appropriate) by the bacteria covered in this volume Yersinia, Shigella, Salmonella, Listeria, enteropathogenic Escherichia coli, and Legionella pneumophila has been studied at the molecular, genetic, and cellular levels. The information derived from these diverse approaches is integrated in this volume into a cohesive picture. In recent years the entry steps have been described at the molecular and genetic level, and the important signal transduction events are still being elucidated. When first studied, the entry steps taken by these diverse bacteria seemed to be quite similar, while the genetic basis of entry seemed to be dissimilar. However, as more has been learned, we find both similarities and differences at all levels. Many interesting questions remain to be answered and these reviews of the "state of the art" provide a foundation from which to proceed.