The need for continuous monitoring of blood gases in critically ill patients has become more and more evident during the past years. Electrochemical sensors appear to be most appropriate for such measurements because of their simplicity, ease of use and low cost. Intravascular sensors and skin surface sensors for pO2 and pCO2 are available today in different degrees of technical maturity. In the case of intravascular sensors, the use of surface heparinisation techniques may lead to considerable functional improvements. In the case of skin surface sensors, a reliable method for simultaneous measurement of cutaneous blood flow is needed. Laser doppler flowmetry appears to be a promising method for this purpose.