In the presence of abnormal cardiac automaticity and conduction, physiologic pacing aims to maintain the heart rate and restore the sequence of cardiac activation. It is assumed that sensors should mimic the behavior of the healthy sinus node response to exercise and nonexercise needs. The atrial electrogram can be used for rate control when sinoatrial function is adequate. A high proportion of pacemaker recipients, however, have either established or progressive abnormal sinoatrial function occurring either at rest or during exercise. Such chronotropic incompetence commonly occurs as a result of medications or ischemia or in association with sick sinus syndrome. In addition, the atrium may be unreliable for sensing or pacing in some patients (such as during atrial fibrillation or paroxysmal atrial tachycardias), and the use of a sensor as an alternative means to simulate sinus node responsiveness is required. These problems prompted the development of nonatrial implantable sensors for cardiac pacing. In addition, the role of sensors has also been expanded to include functions other than rate augmentation, such as the monitoring of cardiac hemodynamics during heart failure and ventricular arrhythmias. Despite the large number of implantable sensors to detect exercise that have been proposed or instrumented in pacing devices, there is as yet no single sensor that can simulate the ideal sinus rhythm behavior.