Since the advent of cardiopulmonary bypass, the generation and elimination of gaseous and solid (micro) emboli have been a concern. Major improvements with respect to gaseous microemboli have been made by the introduction of arterial line filtration and membrane oxygenators. Animal experiments have shown a clear correlation between massive air embolism and outcome. However, limited knowledge is available regarding the cut-off point between the occurrence of negative outcome and the number and size of gaseous microemboli. Generation of gaseous microemboli can occur when using cardiopulmonary bypass. However, no consensus exists on when a given diameter or number of emboli becomes injurious to the patient. An important variable is the gas mixture inside the bubble. Nitrogen has a very long dissolution time that results in a prolonged ischemia for tissue behind the occlusion. The pathophysiologic reaction of the body when exposed to gaseous microemboli is most likely based on ischemia caused by partial occlusion of blood vessels and by endothelial damage. Gaseous microemboli can be cleared mechanically by using filters, by reduction of blood velocity, and by rapid reduction of the nitrogen content. Elimination of gaseous microemboli is dependent on the design of the cardiopulmonary bypass circuit. A membrane oxygenator, although not designed for it, can remove gaseous microemboli. Arterial line filtration is not the best solution for removal of gaseous microemboli, because larger emboli have been fractionated before reaching the arterial filter. Venous line filtration is a more efficient way for clearing gaseous microemboli.