The increasing use of biomaterials in blood-contacting applications underlines the importance of an enhanced understanding of the interactions of blood with foreign surfaces. These interactions constitute a complex response involving the possible participation of proteins, platelets, the intrinsic coagulation, the fibrinolytic system, erythrocytes, leucocytes and the complement system. Interpretation of blood-biomaterial interactions and the development of improved biomaterials require the utilisation of in vitro, ex vivo or in vivo blood compatibility assessment procedures, although there is no ideal procedure for linking assessment to the potential clinical performance of a material. In clinical application, foreign surfaces represent only one of the factors influencing the blood response, with the response also influenced by the presence of antithrombotic agents, the blood condition and the nature of the application.