Blood transfusions are a lifesaving but transient therapy used to correct deficiencies of blood cells and coagulation factors that occur in cancer patients. Anemia can occur in cancer patients as a result of hemolysis, blood loss, or bone marrow failure. The blood component most commonly recommended for the treatment of anemia is packed red blood cells. Coagulation disorders are common with hemangiosarcoma and diffuse hepatic tumors. Fresh frozen plasma is used as a source for replacement coagulation factors for the treatment of disseminated intravascular coagulation or other cancer-associated coagulopathies. Although thrombocytopenia and neutropenia can be the result of bone-marrow failure from tumor infiltration, chemotherapy, or radiation therapy, these platelets and neutrophils are rarely transfused to veterinary cancer patients. Pretransufsion testing consists of blood typing in cats, and cross matching in dogs and cats if the dog has previously been transfused. Cancer patients receiving transfusions should be monitored on a continual basis during and immediately following the transfusion to enable early identification of an adverse event, allowing the transfusion to be discontinued.