Cardiac arrest is the ultimate 911 emergency. The heart stops sending blood to the body. Death occurs in minutes — unless a bystander takes matters into his or her hands and starts cardiopulmonary resuscitation (CPR). This keeps blood circulating until trained and better-equipped first responders arrive on the scene to jump-start the heart back into a normal rhythm. Two new studies in JAMA provide compelling proof that efforts to train people to do life-saving CPR pay off. Many organizations sponsor CPR and AED training programs. Two notable ones are the American Heart Association and the American Red Cross. Many local departments of public health also provide CPR training, including “friends and family” classes for people close to someone at risk of cardiac arrest. The investment of time and effort to learn CPR is small. The potential payoff — saving a life — is huge.