A resuscitation system that includes at least two defibrillation electrodes configured to be applied to the exterior of the chest of a patient for delivering a defibrillation shock, a source of one or more ECG signals from the patient, a defibrillation circuit for delivering a defibrillation shock to the defibrillation electrodes, a control box that receives and processes the ECG signals to determine whether a defibrillation shock should be delivered or whether CPR should be performed, and that issues instructions to the user either to deliver a defibrillation shock or to perform CPR, wherein the determination of whether CPR should be performed and the instructions to perform CPR can occur at substantially any point during a rescue. The control box may include a user operable control for initiating delivery of a defibrillation shock, and the instructions to deliver a defibrillation shock include instructions to activate the user operable control. The user operable control may be a button configured to be pushed by the user. The determination of whether CPR should be performed and the instructions to perform CPR may occur before a determination to deliver any defibrillation shock. The source of the ECG signals may be the defibrillation electrodes. The defibrillation circuit may be contained in the control box.