Both atrial flutter and fibrillation are common arrhythmias in the clinical setting. Although we have been aware of them for decades, little has been known until recently about their triggering or maintenance mechanisms. The diagnosis of these arrhythmias lies largely in the electrocardiogram, which shows characteristic features of atrial electrical activity, leading to a correct diagnosis. Usually, some maneuvers such as adenosine infusion, carotidus sinus massage, etc., are required, in order to unmask the atrial activity, that are often obscured by the QRS complex or T wave. Several therapeutic options can be attempted for the acute termination of both atrial flutter and fibrillation episodes. The choice of one or another depends on some extent, on the clinical status of the patient during the arrhythmia, the presence of structural heart disease and the preceding arrhythmic history. Antiarrhythmic drugs are quite efficacious in the acute conversion of atrial fibrillation, but such an effect is not expected in atrial flutter. Drugs that depress AV nodal conduction can be used in both instances, as a therapeutic end-point or as a previous measure to the arrhythmia conversion. Direct current cardioversion is a good and efficacious option for both arrhythmias, however sedation is mandatory which, may be a contraindication in some patients. Rapid atrial pacing is an elegant and reliable method for the acute termination of atrial flutter of the common type, although a transvenous catheter insertion is needed.