Cardiac arrhythmias routinely manifest during or following an acute coronary syndrome (ACS). Although the incidence of arrhythmia is directly related to the type of ACS the patient is experiencing, the clinician needs to be cautious with all patients in these categories. As an example, nearly 90% of patients who experience acute myocardial infarction (AMI) develop some cardiac rhythm abnormality and 25% have a cardiac conduction disturbance within 24 hours of infarct onset. In this patient population, the incidence of serious arrhythmias, such as ventricular fibrillation (4.5%) ,is greatest in the first hour of an AMI and declines rapidly thereafter. This article addresses the identification and treatment of arrhythmias and conduction disturbances that complicate the course of patients who have ACS, particularly AMI and thrombolysis. Emphasis is placed on mechanisms and therapeutic strategies.