Cardiologists diagnose and treat diseases of the cardiovascular system. Diagnosis involves a detailed history, a physical examination, blood tests, and invasive and noninvasive measurement of cardiac function. Treatment depends on the diagnosis and may involve medical therapy, percutaneous procedures, or surgery (performed by surgeons-not cardiologists). This chapter will deal with hemodynamics from the organ system to the tissue perfusion level and describe the most important physiological measurements required that are important in medical diagnosis, the types of sensors used, and how the signal may be acquired and processed and then displayed. Because the author is a clinically active, academic cardiologist, the relative value and utility of the measurements in the real world will also be addressed. Although engineering and quantitative aspects of the physiology and how the heart works as a coordinated pump will be emphasized, it is important to recall that the entire system we are concerned with is composed of living biological tissue comprised of trillions of living cells. Therefore, it is subject to all the basic biological laws that govern genetics, embryology, cellular physiology, metabolism, inflammation, repair of injured tissue, and so forth, including all of the compensatory pathways that nature has developed through natural selection.