Transfusion is a therapeutic method of intravenous administration of whole blood or blood products. Depending on indications such as anemia, coagulopathy, hypoproteinemia and hypovolemia, targeted therapy with whole blood or blood products (erythrocyte products, blood plasma, platelet concentrate, cryoprecipitate) is chosen. Blood and blood products can be fresh or stored in the refrigerator or freezer. The ideal canine donor should be in good health, mild in nature, with easily accessible veins and between 1-8 years old, with a minimum body weight of 25 kilograms. Cats that meet the criteria for an ideal donor are clinically healthy, with easily accessible veins, aged 1-10 years, weighing over 4 kilograms, without having previously received a transfusion on their own. The blood groups of dogs and cats are defined by speciesspecific antigens found on the surface of erythrocytes. More than 13 blood groups of dogs are known to be categorized in the DEA system, and new blood groups such as Dal, Kai 1, and Kai 2 have recently been discovered. Blood group DEA 1.1 causes the strongest acute hemolytic reactions. Blood groups of dogs are typed by serological cross-reaction tests, gel agglutination tests and immunochromatographic methods. There are 3 blood groups in cats: A, B and AB, but there is also knowledge about new antigens such as Mick. Blood groups of cats are typed by agglutination tests, card systems and immunochromatographic methods. Donor dog blood is collected in open or closed 450 ml bags with anticoagulant. In cats, open blood collection systems are most commonly used, and a total of 50 ml of blood can be taken from an individual cat. The patient should be monitored during and after the transfusion for possible transfusion reactions. Transfusion reactions can be immunological (febrile non-hemolytic, acute hemolytic, allergic / anaphylactic and delayed reactions) and non-immunological (hemolysis, circulatory overload, hypocalcaemia and bacterial contamination).