This review shows that clinical signs cannot be relied on to reveal left-to-right ductal shunting in the preterm. Echocardiography can define many of the haemodynamic features, including occult ventricular dysfunction, and categorize shunts into small, moderate or large, without defining "significance" which is multifactorial. Large shunts have an elevated left atrial:aortic root ratio (>1.3:1 or >1.5:1, depending on fluid and diuretic policy) a ductal diameter >1.4 mm and retrograde diastolic flow in the descending aorta exceeding 30% of the antegrade flow. The neonatal clinician and echocardiographer is reminded to remain wary of occult congenital heart disease, particularly before closing the ductus.