The Purkinje system may play an important role in the initiation and maintenance of ventricular cardiac arrhythmias. In both ventricular tachycardia and ventricular fibrillation, the Purkinje system has been implicated via multiple mechanisms in sustaining these life-threatening rhythms. The need to be able to map the source of these rhythms to the Purkinje system remains an important strategy as a target for catheter ablation. This chapter reviews the current clinical and basic science findings of the complex mapping studies that have recorded from the Purkinje system during arrhythmia. © 2009 Blackwell Publishing Ltd.