The invention concerns a device for non invasive monitoring of the concentration of a constituent of a human or animal bloodstream. In the preferred example the device comprises drive circuitry (50) for provision of an alternating current at a microwave frequency. This frequency is adjustable. The drive circuitry may for example comprise a voltage controlled oscillator. The device has a sensor (19) adapted to be placed in proximity to the body of the human or animal, the sensor being electrically connected to said drive circuitry to receive said alternating current and being adapted to project microwave energy into the said body. Detector circuitry is provided for detecting a signal transmitted and/or reflected by the sensor, the detected signal properties being dependent on the concentration of the said blood constituent.
