One leading cause of death in the United States is sudden cardiac death, often called cardiac arrest or a massive heart attack. Death results when the normally carefully choreographed electrical impulses of the heart become uncoordinated, a condition called ventricular fibrillation. Reversion of fibrillation to any other, more organized, electrical pattern is called defibrillation. Although electrical defibrillation is now widely practiced, the mechanism by which a strong electrical shock can stop fibrillation is still not understood. Defibrillation may fail because it does not extinguish fibrillation or because it creates a new fibrillation. It is not even clear how an electrical stimulus can affect tissue several centimeters away. Another mystery is why there is such a strong dependence on the temporal pattern, or waveform, of energy delivery for defibrillation. For example, it is more effective to withhold some available energy from certain defibrillators. The most recent data on these unknown aspects of defibrillation are discussed.