Cardiac modeling requires knowledge of biology, physiology, physics, and mathematics. This section contains close to 100 interactive applets useful in understanding cardiac dynamics. They range from diffusion of ions and the generation of an action potential to scroll waves (three-dimensional spiral waves). Playing Cyber-cardiologist The applet on the left uses a simple cell model to display electrical waves in the heart during simulated normal sinus rhythm, ventricular tachycardia, and ventricular fibrillation. Click the buttons to select a rhythm or to apply a defibrillating pulse to one of the arrhythmias. It is also possible to induce electrical activations by clicking directly over the heart surface. The size of the stimulus is controlled by changing the size in the "stimulus size" window and pressing return. At startup the heart is in ventricular fibrillation.