While some research requires that dogs and cats are used, the vast majority of laboratory animals are rodents specifically bred for research. Nearly half of the dogs and cats needed for research are also bred for that purpose. Since state laws and local policies prevent many animal pounds and shelters from providing dogs and cats to research facilities, animal dealers are the primary source for the other half of the animals scientists require. These dealers must be licensed by the United States Department of Agriculture (USDA) and must adhere to Animal Welfare Act standards of care.

There are striking similarities between the physiological systems of humans and various species of animals. For example, much of what we know about the immune system has come from studies with mice, and much of what we know about the cardiovascular system has come from studies with dogs.