Although first described about 100yr ago, atrial fibrillation (AF) is now recognized as the most common of all arrhythmias. It has a substantial morbidity and presents a considerable health care burden. Improved diagnosis and an ageing population with an increased likelihood of underlying cardiac disease results in AF in more than 1% of population. AF is associated with an approximately two-fold increase in mortality, largely due to stroke which occurs at an annual rate of 5-7%. Another risk to survival is heart failure, which is aggravated by poor control of the ventricular rate during AF. Usually AF is associated with a variety of symptoms: palpitations, dyspnea, chest discomfort, fatigue, dizziness, and syncope. Paroxysmal AF is likely to be symptomatic and frequently presents with specific symptoms, while permanent AF is usually associated with less specific symptoms. However, in at least one third of patients, no obvious symptoms or noticeable degradation of quality of life are observed. This asymptomatic, or silent, AF is diagnosed incidentally during routine physical examinations, pre-operative assessments or population surveys. Recently, a very large incidence of generally short paroxysms of AF has been seen in patients with implantable pacemakers or defibrillators and these arrhythmias are often silent. Pharmacological suppression of arrhythmia may be associated with a conversion from a symptomatic to an asymptomatic form of AF. Holter monitoring and transtelephonic monitoring studies have demonstrated that asymptomatic episodes of AF exceed symptomatic paroxysms by twelve-fold or more. Although symptoms may not stem directly from AF, the risk of complications is probably the same for symptomatic and asymptomatic patients. AF is found incidentally in about 25% of admissions for a stroke. Studies in patients with little or no awareness of their arrhythmia condition indicate that unrecognized and untreated AF may cause congestive heart failure. In patients with coronary bypass, AF may not only represent risk for immediate postoperative morbidity and increase hospital resource utilization, but being unrecognized, may produce a significant impact on long-term survival and quality of life. Although silent AF merits consideration for anticoagulation and rate control therapy according to standard criteria, whether antiarrhythmic therapy is relevant in this condition remains unclear.