Arrhythmia is any deviation from or disturbance of the normal heart rhythm. Arrhythmias may be benign, symptomatic, life threatening or even fatal. Their consequences depend not only on their manifestations but also on the presence of important abnormal structural conditions of the heart. The antiarrhythmic agents have been traditionally divided into four distinct classes on the basis of their mechanism of action. The Chapter includes various methods of inducing arrhythmias, in vitro and in vivo methods for testing the antiarrhythmics.