Members of genera belonging to the Enterobacteriaceae family have earned a reputation placing them among the most pathogenic and most often encountered organisms in clinical microbiology. These large Gram-negative rods are usually associated with intestinal infections, but can be found in almost all natural habitats. They are the causative agents of such diseases as meningitis, bacillary dysentery, typhoid, and food poisoning. As well as being oxidase negative, all members of this family are glucose fermenters and nitrate reducers. In most cases, the pathogenicity of a particular enteric bacterium can be determined by its ability to metabolize lactose. Non-utilizers are usually pathogenic while the lactose utilizers are not. Because many different species in this family can cause similar symptoms, biochemical tests are crucial to the identification, diagnosis, and treatment of infection. We will discuss the twelve genera of the Enterobacteriaceae family which are most commonly encountered in the clinical laboratory: