Continuous blood glucose monitoring aims to: better evaluate glycaemic variations; better detect hypoglycaemia; and, ultimately, automatize insulin delivery (artificial beta cell). The sensors can be fully implantable, with the challenge of constructing durable systems to avoid repeated implantations. In-dwelling needle-like electrodes and microdialysis fibres with a pump that brings the dialysate to the glucose sensor are inserted in the subcutaneous tissue through the skin. The GlucoWatch is an almost non-invasive technique that extracts the extracellular fluid by iontophoresis. In these systems, the glucose oxidase generates the electrical signal, proportional to the glucose concentration. Non-invasive techniques aim at measuring the glucose concentration without breaching the skin, using absorption of light in the infrared spectrum. These techniques have not reached the necessary reliability for use as glycaemic alarms, and even less as artificial beta cells. Currently, glucose sensors are mainly used as glycaemic holters to help in the management of insulin therapy.