Atrial fibrillation (AF) is one of the most common arrhythmia complications of acute coronary syndrome (ACS). The incidence of new-onset AF is 2.3-37 %, and it is an important predictor of a patient's morbidity, mortality, and prolonged hospitalization. Various risk factors for the development of new-onset AF after ACS have been identified, including: old age, higher Killip class, relevant history (e.g., hypertension), and enlarged left atrium. Insights into the pathophysiological mechanisms of new-onset AF have been provided by both experimental and clinical investigations and show that new-onset AF is multifactorial, involving atrial ischemia and atrial stretch, inflammation, autonomic nervous system activity, and hormone activation. An understanding of the mechanisms underlying new-onset AF complicating ACS can provide new insight of therapeutic importance.