The incidence of complete heart block (CHB) following open-heart surgery for congenital heart disease is about 1%. Most of postoperative CHBs are the consequence of procedures involving the closure of ventricular septal defect; they usually occur immediately after surgery or early in the postoperative period; in few cases they also may occur many months or years after surgery. Early postoperative CHB can be transient or permanent. Permanent pacing is generally not recommended in the former. On the contrary, if CHB persists after at least two weeks of temporary pacing, permanent pacing is needed because the block is usually due to His bundle damage or to trifascicular damage and this is associated with excessive bradycardia and risk of asystole. Late postoperative CHB can be due to the recurrence of previous transient early postoperative CHB or to the progression of postoperative His-Purkinje conduction troubles suggesting trifascicular damage. Permanent pacing is obviously needed in case of documented late postoperative CHB. The prophylactic use of permanent pacing in patients at risk of late postoperative CHB is still a controversial point. Electrophysiologic studies should be performed in such patients. The occurrence of second degree AV block within or below the bundle of His during atrial pacing at rate lower than 200/min can be considered a good marker of impending CHB. In this case prophylactic permanent pacing should be recommended, especially in patients with coexisting problems of troublesome or malignant tachyarrhythmias who have to be treated with antiarrhythmic drug therapy that may favour the progression to CHB.