Abstract. Blood transfusion has become a universally accepted, life‐saving procedure in modern clinical medicine. In addition, specific blood fractions are widely used in the therapeutic treatment of haematological disorders. Problems are, however, encountered in conventional transfusion practice and in the clinical use of blood components. This paper outlines some of those problems and considers how plasma expanders and oxygen‐carrying blood substitutes may be used to overcome some of them. The extent to which acceptable blood substitutes have been developed and tested in both animal and human studies is especially emphasized.