The development of glucose sensors for clinical use is driven by the aim of blood glucose normalization in diabetic patients, and the detection of blood glucose impairment in nondiabetic subjects submitted to critical conditions. Currently available devices for continuous glucose monitoring are either minimally invasive or partly implantable, and measure subcutaneous interstitial glucose using glucose-oxidase. The signal generated is calibrated against capillary blood glucose to provide estimated blood glucose values. The three certified glucose sensing systems for clinical use: GlucoWatch(R) biographer, CGMS(R) and GlucoDay(R) use different techniques for glucose recovery: reverse iontophoresis, direct subcutaneous implantation and microdialysis, respectively. These devices show sufficient accuracy to identify trends of blood glucose variations, but provide information either with significant delay (GlucoWatch(R)) or retrospectively after computerized analysis of "Holter-style" glucose recordings (CGMS(R), GlucoDay(R)). Only GlucoDay(R) can provide almost online data. Occasional use for short-term assessment of glucose control or educational purposes in diabetic subjects dominates the present clinical use of these devices. Because of the discrepancies between variations of blood and interstitial glucose levels, treatment adaptations from collected sensor data still need further assessment by capillary blood glucose measurements, while the limited biocompatibility of these systems allows only short-term recordings for a few days at most. Subcutaneous and intravenous, fully implantable, sensor models are currently under development and evaluated in clinical trials. Designed for long-term glucose sensing and permanent clinical use, these glucose sensors show promising early results in view of open-loop or closed-loop insulin delivery based upon a sensor signal.