Automated external defibrillators (AEDs) are machines that are programmed to safely deliver an electrical shock to a person who is in cardiac arrest. In cardiac arrest, the heart suddenly stops beating. The AED checks the heart. Then a shock may be given to help the heart start beating normally again. Each AED has instructions for that machine.

AEDs are often placed in public buildings in visible areas. Before an AED is used, all the steps for CPR must be followed. The AED has a computer inside that will "speak" instructions for when to continue CPR and when to check for a heart rhythm (pulse). The AED must be placed next to the person who has collapsed. Then it must be turned on, and all the commands must be followed.