The CGMS (Continuous Glucose Monitoring System) is a portable device allowing continuous measuring of glucose. It provides recordings of at least 72 h, during which 288 measures/day are performed. Results are visualised in the form of a set of curves, illustrating the variations in blood glucose levels over time. The quality of the records has often been questioned by several authors. Some of the system's physiologically related limitations can be explained by the less than perfect coincidence of variations in glucose levels observed in the interstitial tissue, where CGMS measurings are done, and in the blood, where calibrations are performed. Other limitations, such as defects in accuracy or in reproducibility of tracings or premature curtailments of recordings, are due to technical weaknesses which were considerably improved during the past few years, particularly with regard to the quality of the electrodes providing a more stable signal over time. In clinical practice, CGMS is a tool for investigating the glycaemic patterns of diabetic patients in conjonction with SMBG. It allows the identification of overlooked hyper- or hypoglycaemia. Generally well accepted, it is a usefull tool to analyse the nocturnal period, or any situation where glucose checks are rare. The visual nature of its results provides a facilitating support in the discussion between the patient and the care-provider during consultations or educational sessions. CGMS utilisation was proposed for guiding treatment adjustment. At present, it is still difficult to state with certainty that this tool allows effective improvement in the metabolic control of patients with type 1 diabetes, in view of the paucity of controlled studies showing an impact on HbA1c values or on the frequency of hypoglycaemia, even if such a tendency emerges from most non-controlled intervention trials.