‘Blood transfusion’ outlines the history of transfusing animal blood dating back to the 17th century. The 19th century saw the first successful human blood transfusion, but two major issues remained: the problems of clotting and blood group incompatibility. Albert Hustin and Luis Agote resolved the first issue in 1914 by using sodium citrate in transfusions to work as an anticoagulant. Richard Lewisohn calculated the correct levels of citrate needed to avoid poisoning the blood. Karl Landsteiner’s work in early 20th-century Vienna revealed the ABO blood type distinctions, solving the latter problem. The creation of blood banks and the potential for viral contamination of blood and blood products are also discussed.