AbstractMyocardial stunning can follow regional or global ischemia in the adult or immature heart. This report reviews some of our studies of the protean causes of stunning including energy and substrate depletion, inefficient oxygen utilization, calcium loading, acidosis, oxyradical damage, and summarizes studies and strategies to limit its occurrence. Data showing that reintroduction of molecular oxygen during reoxygenation of immature cyanotic hearts can cause stunning via a newly described biochemical pathway involving superoxide anion and nitric oxide are also included.