Devices, systems, and methods for leadlessly stimulating the heart. Through a magnetic signal generator positioned outside or inside the thoracic cavity, a magnetic signal is transmitted through the chest to stimulate electrical activity within the myocardial muscles. The magnetic signal may function as a pacemaker, cardioverter or defibrillator. Advantages of magnetic stimulation include, without limitation, non invasiveness, a reduction or even elimination in pain, and access to tissues covered by poorly conductive structures.