Measurements of central venous oxygen saturation (ScvO2) have been successfully used to guide haemodynamic therapy in critical care. The efficacy of this approach in the treatment of severe sepsis and septic shock has stimulated interest in the use of ScvO2 to guide management in patients undergoing major surgery. The physiological basis of ScvO2 measurement is complex. A number of outstanding issues will need to be resolved before incorporating ScvO2 measurement into routine practice. First, it is not yet clear which value of ScvO2 should be targeted. Second, there is some uncertainty as to which interventions are the most effective for achieving the desired value of ScvO2 or how long this value should be maintained. The study by The Collaborative Study Group on Perioperative ScvO2 Monitoring published in this edition of Critical Care may help provide answers to some of these questions. Our understanding of ScvO2 measurement remains limited, however, and the routine use of peri-operative ScvO2-guided goal-directed therapy cannot be recommended until a large randomised trial has confirmed the value of this approach.