ECG, 24-h ambulatory ECG, and related tests have been used to clarify the pathophysiological features of atrial fibrillation as well as to detect the relationship between symptoms and episodes of tachyarrhythmia. These testes are also potent to estimate the efficacy of pharmacological and nonpharmacological treatments. Proarrhythmic events during the treatment with antiarrhythmic agents may be avoided by careful follow-up using ECG. Recently, an implantable loop recorder electrocardiogram is available to identify the culprit episode of syncope. Atrial fibrillation is a cause of syncope in only a small proportion of patients.