An automated external defibrillator (AED) is a device that can detect and correct a specific type of abnormal heart rhythm called ventricular fibrillation. Ventricular fibrillation causes cardiac arrest. If cardiac arrest occurs, an AED, if available, should be used immediately. An AED is used before calling for help and before attempting cardiopulmonary resuscitation (CPR) because an AED is more likely to save lives. If the AED detects ventricular fibrillation, it provides an electrical shock (defibrillation) that can restore normal heart rhythm and start the heart beating again. Emergency medical care should be obtained even if the heart has started beating again. If a person remains in cardiac arrest after an AED is used, CPR should be done. AEDs are easy to use. The American Red Cross and other organizations provide training sessions on the use of AEDs. Most training sessions take only a few hours. Different AEDs have somewhat different instructions for use. The instructions that are written on the AED being used should be carefully followed. AEDs are available in many public gathering places, such as stadiums and concert halls. People who are told by their doctor that they are likely to develop ventricular fibrillation but who do not have an implanted defibrillator may want to purchase an AED for home use by family members.