This chapter discusses the cardiac cycle, where the activity of the heart follows a cyclic pattern. One contraction and one relaxation together form a cardiac cycle. Several mechanical, physical and electrical events are associated with the cardiac cycle. Atrial depolarization is associated with atrial contraction. As soon as the ventricular pressure rises above the atrial pressure, the atrioventricular valves snap shut. The rise in aortic pressure is transmitted to all major peripheral arteries as the arterial pulse. Closure of the semilunar valves gives rise to the second heart sound. Sudden changes in flow create vibrations which can be heard. Phonocardiographic records give the exact duration and frequency of each sound, which is impossible to obtain with a stethoscope. Cardiac catheterization is a highly versatile technique of immense value in physiological studies as well as management of heart disease.