Under normal conditions, the function of Purkinje fibers is fast conduction of impulses originating in the sinoatrial node, in order to secure a synchronous ventricular contraction. However, when the activation from the atria fails to activate the Purkinje network, the pacemaker activity of Purkinje fibers is physiologically responsible for the initiation of the idioventricular rhythm. The functions of Purkinje fibers require a set of characteristics suitable for fast conduction and pacemaker activity, such as a large and fast upstroke and diastolic depolarization, respectively. The aim of this chapter is to present the ionic events that are responsible for the action potential and diastolic depolarization of Purkinje fibers. Furthermore, some mechanisms of recovery from excitation are considered, such as the extrusion of Na+ by the Na+-K+ pump and of Ca2+ by the Na+-Ca2+ exchange. The sympathetic and frequency-dependent control of Purkinje fiber automaticity is discussed as well. The ion fluxes associated with electrical activity in systole and diastole as well as the events involved in recovery from excitation can become deranged under a variety of conditions. Therefore, the characteristics and consequences of derangements of the events responsible for excitation and recovery from excitation of Purkinje fibers are discussed and their mechanisms analyzed. This approach intends to provide insights as to the manner by which derangements of physiological mechanisms lead to disturbances of cardiac function, often in the form of life-threatening arrhythmias.