Cardiac arrhythmias are commonly seen in critically ill patients. If untreated, these arrhythmias can often lead to patients becoming hemodynamically unstable and may result in sudden cardiac arrest. Management strategies, both pharmacologic and non-pharmacologic, are based on the presenting arrhythmia and the hemodynamic status of the patient. In this chapter, various atrial and ventricular arrhythmias are defined and discussed along with their respective treatments as well as an overview of the various antiarrhythmic classes.