Accurate evaluation of an atrioventricular septal defect is readily achieved by echocardiography. A sound understanding of the basic morphology and associated lesions is key to this approach. This article first details the features that are common to all hearts with an atrioventricular septal defect, irrespective of the presence or absence of an interatrial or interventricular communication. These common features are: (1) inlet outlet disproportion; (2) absence of the atrioventricular muscular septum; (3) abnormal position of the left ventricular papillary muscles; (4) abnormal configuration of the atrioventricular valves and, (5) cleft in the left atrioventricular valve. These are all predicated by a sprung atrioventricular junction. Second, is a detailed outline of the associated risk factors that must be identified by the echocardiographer prior to presenting the patient for surgical management, with the most important ones being abnormalities of the left atrioventricular valve and left ventricular outflow tract obstruction. Indeed, in this current era it is rarely necessary to perform other investigations prior to surgical repair.