Pacemakers are implantable devices designed to ameliorate symptomatic slow heart rates. The first implanted pacemakers were developed fifty years ago and were large, unreliable devices with short lifespans. Pacemakers have evolved from devices that only paced the ventricle from a single site to sophisticated devices that utilize one, two, or three leads and that maintain atrioventricular synchrony, attempt to mimic normal ventricular activation, and continuously monitor the heart rhythm and other hemodynamic parameters.