Cardiac arrhythmias may pose a significant haemodynamic threat to the paediatric patient. Not only extremes in heart rates and/or the loss of atrioventricular (AV) synchrony can greatly compromise the cardiac output, but sustained tachycardias can also lead to heart failure and cardiomyopathy in an otherwise healthy child. The immediate postoperative period is an especially vulnerable time for the development of arrhythmias, and therefore, the paediatric intensivist should be comfortable in their recognition and initial management. This chapter will offer a general approach to the diagnosis and treatment of the most commonly seen cardiac arrhythmias in the PICU.