The use of blood and blood products in cardiac surgery has undergone marked changes over the past three decades. This revolution occurred primarily through the study and understanding of hemodilution. This review marks the changes in the practice of blood transfusion in cardiac surgery. We review the methods of blood conservation through preoperative screening and preparation, intraoperative drug therapy and modification of surgical techniques, and postoperative use of shed mediastinal blood. We also examine the physiologic effects and limitations of hemodilution in adult and pediatric cardiac surgery. Finally, we review the recent trends in the use of blood component therapy and the future of blood substitutes.