The skill and experience of the medical and nursing staff are the factors that have the greatest impact on the quality of monitoring and treatment of the critically ill surgical patient. Basic determinations at the bedside together with periodic evaluation of the whole patient by the medical staff may, in selected cases, be supplemented usefully by more invasive monitoring techniques. The specific complications and technical pitfalls of these techniques should be known, and caution should always be exercised that the values provided are not misinterpreted. A critical care area serving a major teritary referral hospital should be able to measure pulmonary capillary wedge pressure and cardiac output. Mixed venous oxygen content and arterial lactate concentration, as indices of oxygen delivery, are also useful measures.