A blood glucose monitor for non-invasive, in-vivo characterisation of a blood glucose level in a living body, the monitor comprising: a microwave resonator (12) having a resonant response to input microwaves and designed such that said response will experience a perturbation by a living body in proximity or contact with the resonator and detection means for detecting changes in said resonant response from which said level can be characterised. The microwaves are resonated at a frequency determined by the resonant ring (34, 36), the device may comprise two rings (34, 36) which resonate at different frequencies. One frequency may be susceptible to change due to proximity with a living body and the other may not, allowing for correction for systematic errors.
