This chapter talks about the cardiac arrhythmias in ICU. Atrial fibrillation (AF) and atrial flutter are the most common pathological supraventricular arrhythmia in the ICU setting. AF is seen in the setting older age, male sex, hypertension, ischemic heart disease, thyrotoxicosis, alcohol binging and pulmonary embolism. Atrial flutter is an organized regular rhythm, results usually from a re-entrant circuit around the tricuspid valve in the right atrium. Multifocal atrial tachycardia (MAT) is also seen in patients with congestive heart failure and theophylline toxicity. Ventricular premature complexes and ventricular tachycardia are ventricular tachyarrhythmias. Normal conduction system, sinus node abnormalities, atrioventricular block, atropine, temporary pacemakers are the bradyarrhythmias and conduction block. Waves and intervals of normal electrocardiogram (ECG), basic pathophysiology of tachyarrhythmias, normal conduction system and mechanisms of bradyarrhythmias are also discussed in this chapter.