The Priming volume of the oxygenators has been reduced by 75% during the last two decades and it is now possible to go safely on bypass with a priming volume less than 150 ml. This small volume reduces hemodilution and need for blood components. It will decrease the inflammatory reaction especially in neonates and enhance coagulation after CPB. Nevertheless it is still difficult to avoid packed red cells for neonatal CPB with patients less than 3 kg. For older children, many techniques (autologous retropriming, vacuum assisted venous drainage, cell saver ultrafiltration) allow to prime the circuit with clear solutions. Continuous monitoring of SvO₂, hematocrit and régional oxygenation index facilitate the management of CPB with small priming volume. The reduction of the priming volume must always take in account the safety of the children.