**1. Introduction**

Although ICD therapy is an efficient and reliable therapeutic method, internal defibrillation is a traumatic experience; moreover and there is accumulating evidence that internal defibril‐ lation, especially with high defibrillation energy, may adversely affect cardiac function and even patient prognosis [1, 2]. Therefore, considerable research efforts have been focused on better defining defibrillation mechanisms, particularly aiming at and improving defibrillation efficacy and in order to reduce lowering defibrillation energy requirements.

Recent developments in cardiac simulation have been used to create more accurate defibril‐ lation models. The chapter tries to focus on the importance of geometry in the different defibrillation models, and explores potential applications of these models to improve the transvenous defibrillation systems, and the more recently developed extracardiac ICDs.
