Accurate and reliable devices sensing glucose on a (near)-continuous basis may facilitate specific therapeutic adjustments that need to be made to avoid hypo- and hyperglycaemic excursions, thereby improving metabolic control. Current continuous glucose monitoring (CGM) systems indicate the glucose level, the direction and magnitude of change of glucose levels, and can be used to assess glycaemic variability. In addition, real-time CGM sensors can serve as a tool to predict impending glucose excursions, thereby providing alarm signals of hypo- and hyperglycaemic values warning the patient to take preventative actions. Quality of life may also improve by using CGM via reducing the fear of hypoglycaemia. Particularly patients with brittle diabetes, hypoglycaemia unawareness, gastroparesis, pregnant women, or pump users, who are motivated to participate in their diabetes care and are technologically adept, may benefit from CGM. However, to successfully implement CGM in daily practice, these devices must be accurate and reliable, and one must be aware of the limitations of current CGM systems, that originate from physiological and technical aspects. Whether CGM succeeds in improving metabolic control, reducing hypoglycaemic episodes, and improving quality of life in the majority of patients remains to be proven. Should this be the case, real-time CGM may reduce chronic diabetic complications, and avoid hospitalisations, thereby reducing health care costs. In this article we will review indications, advantages, limitations, clinical and technical aspects of current minimally-invasive and non-invasive CGM sensors.