Ultrasound evaluation of the venous system is now a compulsory part of the haemodynamic assessment of the fetus. Once umbilical venous flow was introduced1,2 and its pulsatile pattern discovered in the compromised fetus,3 other sections of the venous system have been added or explored for possible diagnostic use: the inferior and superior vena cava,4,5 ductus venosus,6,7 hepatic veins,8 pulmonary veins,9,10 and intracranial veins.11-13 The following presentation is not intended to be a complete review of the fetal venous circulation, which is growing by the day, but rather to focus on some central issues with an emphasis on physiologic principles. The reason for this focus is that, as clinicians, we tend to work according to pattern recognition, which is a necessary principle in daily life. However, in the long run as the fetal patient increasingly demands a more dynamic approach to solve the diagnostic riddles, we find ourselves digging deeper into the physiological mechanisms behind ultrasound images and recordings.