Atrial fibrillation (AF) is a common clinical problem, particularly in the elderly and in patients with organic heart disease. AF generally is classified into paroxysmal and chronic forms. Chronic AF can be the end result of paroxysmal AF in about 30% of patients. Paroxysmal AF can be defined as attacks of arrhythmia lasting < 7 days separated by prolonged periods of sinus rhythm. Chronic AF is AF established for > 7 days. Therefore, the differentiation of paroxysmal from chronic or established AF is based on the history of recurrent episodes and the duration of the current episode of AF. The first episode of persistent AF or the first discovery of AF often is referred to as recent onset AF. Most epidemiologic studies highly underestimate the incidence of paroxysmal and/or asymptomatic AF. The prevalence of AF varies with the age group and patient population studied. AF is found in 3% to 5% of the population > 60 years of age. AF is associated with organic heart disease in 70% to 80% of patients. Of the patients admitted to our Cardiology Division during 1 year, 15% of hospitalized patients had a documented history of AF. The risk of an individual patient developing AF often is difficult to assess, but increasing age and the presence of valvular heart disease and congestive heart failure increase the risk of AF. Other important predictive and causative factors of AF include hypertension, diabetes in women, left ventricular hypertrophy in both sexes, and coronary artery disease, mainly in older patients and patients with left ventricular dysfunction. Other causes of AF include coronary artery disease, hypertrophic cardiomyopathy and dilated cardiomyopathy, chronic obstructive pulmonary disease, pericarditis, and congenital heart disease such as left atrial myxoma and atrial septal. AF can occur in the absence of detectable organic heart disease, so-called "lone AF," in about 30% of cases. The term "idiopathic AF" implies the absence of any detectable etiology including hyperthyroidism, overt sinus node dysfunction, and overt or concealed preexcitation. Stroke is the most important factor of mortality and morbidity associated with AF. These epidemiologic data are essential for designing appropriate therapeutic treatment of this common arrhythmia.