Atrial fibrillation (AF) is the most common observed cardiac arrhythmia and is the most frequent condition associated with thromboembolic events in patients with or without mitral valve disease. The source of cardiac emboli is the left atrium, the left atrial appendage or, less frequently, the left ventricle. Emboli may also originate from aortic atherosclerotic plaques. It is important to identify patients at risk in order to perform the appropriate therapy. Risk stratification is multiparametric, being based on clinical, laboratory, and echocardiographic data. Several trials have pointed out the role of echocardiography in the evaluation of anatomic and functional parameters associated with thromboembolic risk. Transthoracic echocardiography (TTE) does not provide sufficient information regarding posterior cardiac structures, being its sensitivity in detecting thrombi relatively low (33-72%). Transesophageal echocardiography (TEE) in contrast, has an almost 100% sensitivity; this technique is, therefore, mandatory in patients with AF for an adequate prevention of thromboembolism. The echocardiographic information joined with clinical features allow to stratify, in a proper way, the risk of every single patient.