Arrhythmia is a group of conditions where the heartbeat is irregular, too slow, or too fast. Rapid heartbeats or an irregular cardiac rhythm may accompany most forms of heart disease. The process is often benign, but potentially lethal arrhythmias must be assessed carefully and treated to prevent cardiac events. This chapter focuses on the diagnosis of arrhythmias, treatment of arrhythmias, management of narrow QRS regular tachycardia, management of narrow QRS irregular tachycardia, ablation, multifocal atrial tachycardia, management of wide QRS regular tachycardia, and management of wide QRS irregular tachycardia. Ablation has a role in patients who have had more than three episodes of paroxysmal atrial fibrillation and particularly in those with rapid rates more than 150 beats/min.