Escherichia coli is a commonly encountered commensal of the lower intestinal tract of humans and other mammals. Strains of the species are responsible for a significant amount of human morbidity and mortality each year. Consequently, numerous efforts attempt to track the movement of hospital-acquired infections, determine the source of a foodborne disease outbreak, or investigate the seasonal patterns of pathogen abundance in domestic animals. All of these endeavors require that the isolates acquired be differentiated from each other in some manner. This review briefly describes some of the commonly used molecular typing methods for E. coli. However, the main aim of the review is to describe the many levels, from the species to individual strains, at which E. coli can be considered, and to contend that a hierarchical approach to strain typing may often reveal patterns that are not obvious when a typing scheme is simply designed to differentiate isolates.