To describe the principles underlying oesophageal Doppler monitoring of cardiac output, and how invaluable information relating to left ventricular contractility, preload and afterload can be simply derived from the shape of the flow velocity waveform.This review highlights numerous applications in critical care and summarizes the nine prospective, randomized perioperative fluid optimization studies guided by Doppler ultrasonography showing outcome benefit. The significance of these findings was recently endorsed in separate analyses by the United States Agency for Healthcare Research and Quality and the UK Centre for evidence-based purchasing.A clear role is emerging for minimally invasive monitoring of blood flow and haemodynamics using oesophageal Doppler. As with any monitoring modality, it has to be incorporated into an appropriate and rational management stratagem. Its safety record, rapid learning curve, reliability and ease of use encourage further application.