This chapter discusses Enterobacteriaceae, which consist of a large, heterogeneous group of aerobic gram-negative bacilli whose natural habitat is the gastrointestinal tract of animals and humans. It highlights three members of the Enterobacteriaceae that are considered pathogens whenever they are isolated from humans: namely Salmonella, Shigella, and Yersinia. It describes the unique clinical features of the pathogens, which cause significant community- and hospital-acquired infections. Escherichia coli is the most frequently isolated pathogen, and it has been noted to have a unique pathogenesis in terms of its ability to cause urinary tract infections. This pathogenesis involves adherent fimbriae and production of glycocalyx, which allow E. coli to adhere to normal bladder epithelium and invade bladder cells, where it is able to survive in biofilm colonies.