Open heart surgery is one of the most highly technical of all modern medical techniques, and includes procedures such as coronary artery bypass grafting, cardiac valve repair or replacement, correction of congenital defects, resection of aneurysms, ablation of abnormal pathways of conduction, etc. It relies on the coordinated interaction of a heart surgeon, an anesthesiologist, several nurses and technicians, and a perfusionist. The first successful open heart surgery was performed in Philadelphia forty years ago by Dr. John Gibbon, Jr., whose wife, Mary, was his perfusionist. This historical landmark came after two decades of laboratory exploration and perfection of their extracorporeal circuit and its ability to sustain life. Perfusion, the technology which has evolved from those groundbreaking discoveries, controls, supports and maintains the circulation by application of extracorporeal devices. During open-heart surgery, perfusion (cardiopulmonary bypass - CPB) supplants the functions of the heart and lungs to provide the surgeon with a still, dry operating field. Today, this highly specialized role is performed by individuals conversant in a variety of scientific modalities working in close communication and cooperation with the surgeon. Perfusionists understand the anatomy, pathology, and physiology of the patient, while administering medications, anesthetics, blood, blood components and blood substitutes. Simultaneously, they operate a highly sophisticated electromechanical device to substitute for the human heart and lungs. Today's perfusionists know and utilize aspects of varied pursuits which include a functional comprehension of machines and motors, electronics and electrical safety, plastics and biocompatibility, drugs and pharmacology, blood and its components, hemodynamics and fluid dynamics, hypothermia and hyperthermia, gas exchange and metabolism, electrolytes and blood compatibility, anticoagulation and anesthesia. The logarithmic expansion in these unrelated fields of study have enhanced our ability to provide patient care.