Dogs, cats, birds, and ferrets are popular companion animals. Because these pets are considered by many to be family members, they are provided high-quality veterinary medical care, including blood transfusions. This article reviews the current status of blood groups in dogs, cats, birds, and ferrets and discusses the impact of blood groups on veterinary transfusion medicine. One blood group with 3 types has been described in the cat, whereas multiple blood groups have been described in the dog. Only rudimentary knowledge exists regarding pet bird blood groups, and, to date, the ferret appears to be unique because no blood groups have been described. Antibodies against blood group antigens also play a role in animal blood transfusions. Cats have naturally occurring alloantibodies; however, dogs do not appear to have clinically significant naturally occurring alloantibodies. Understanding the issues related to blood groups and blood group antibodies in companion animals will also benefit those using these species as research models for human diseases.