Recent progress in the surgical treatment of malformations after transposition has been dealt with has been made in three directions: 1. By knowledge of the position of the conducting pathways in relation to ventricular septal defects, so that total atrioventricular block, the first complication of this type of surgery, may be avoided; 2. By appreciation of the proximity of the conductive pathways to the elements contributing to pulmonary stenosis, which may contra-indicate direct removal of such stenosis. In such a case, a valved tube must be placed between the sub-pulmonary ventricle and the pulmonary artery; 3. By appreciating the frequency, the difficulty in diagnosis and the grave consequences of failing to recognise lesions of the atrioventricular valves, with the result that their systematic investigation should form part of the operative technique.