Automatic external defibrillators (AEDs) are known to improve outcomes after out-of-hospital cardiac arrest when used by trained public-safety personnel. This study showed that the same is true when AEDs are used by trained laypersons in public locations, such as shopping malls and recreational facilities. Automatic external defibrillators (AEDs) improve outcomes when used by trained laypersons in public locations. Sudden out-of-hospital cardiac arrest is a leading cause of death and disability and a leading source of health care costs in the United States. 1 , 2 When out-of-hospital cardiac arrest is caused by ventricular fibrillation, defibrillation is an effective treatment; however, its effectiveness diminishes with each passing minute. 3 , 4 Automated external defibrillators (AEDs) are safe and effective when used by trained public-safety personnel who have a duty to respond to medical emergencies. 5 – 10 However, it is unclear whether trained volunteer laypersons who do not have a duty to act could save additional lives by using AEDs in addition to cardiopulmonary resuscitation . . .