For much of medical history, the precise mechanism of sudden cardiac arrest was assumed to be the abrupt cessation of activity in diastole. Not until the nineteenth century was ventricular fibrillation recognized as the causative entity, and not until the late twentieth were practical electrical defibrillators developed for terminating this dysrhythmia. Timely defibrillation is considered by many to be the most important single factor in successful cardiac arrest resuscitation. The likelihood of survival is directly related to the duration of cardiac arrest and the time that elapses before the first defibrillation attempt occurs. Over the past two decades, automatic external defibrillators have been developed and improved to provide more rapid defibrillation for victims of cardiac arrest. Their development has been considered the most important recent advance for improving outcome in out-of-hospital sudden death. The internal microprocessor-based system contained in the automatic defibrillator detects VF and prepares the machine to deliver a shock. It has been recommended that these devices be made available for use by basic rescue responders, lay bystanders with a minimal amount of training, and family members of high-risk patients. Public access defibrillation is increasingly seen as a logical extension of the automatic external defibrillator concept.