Required flow rates for cardiopulmonary bypass depend on the patient’s body surface area and temperature. At 37°C, flow of 2.2 L/m2 per minute is required for adequate perfusion. Oxygen consumption is reduced, however, by 50 per cent for every 10°C drop in temperature. At 20°C, a 30-minute period of circulatory arrest can be safely tolerated. A guide to the adequacy of tissue perfusion can be achieved by measurement of the mixed venous oxygen saturation. Some degree of hypothermia is used in most operations, because this provides a margin of safety in the event of unexpected cessation of flow. Although arterial perfusion pressure does not necessarily equate to tissue perfusion, a perfusion pressure of greater than 50 mm Hg is recommended. Higher perfusion pressures are used in the presence of generalized atherosclerosis, especially when renovascular or cerebrovascular disease is present. It is imperative that the surgeon has a sound working knowledge of the bypass circuit, a specific plan for the conduct of cardiopulmonary bypass during the procedure, and clear communication with the other members of the operative team.