Until recently the term “genetic basis of antibiotic resistance” was interpreted narrowly to mean the properties of the genes that confer on a bacterium the ability to resist the action of one or more antibiotics. Scientists who work on antibiotic resistance now realize, however, that this perspective is too narrow. Just as important as the properties of the resistance genes themselves is the ability of bacteria to transfer these genes to other bacteria. Antibiotic resistance is increasingly seen as an ecological problem, which includes the ecology of resistance genes as well as the ecology of the resistant bacteria themselves.