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 having a resonant response to input microwaves and designed such that said response will experience a perturbation by the 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; wherein: a. the resonator comprises a housing defining a space with a microwave-transparent window to which said living body can be offered, the housing including a microwave input to the space and a microwave output from the space; b. the detection means are configured for recording a power versus frequency spectrum of microwaves that have passed through the resonator; c. the monitor further comprises computing means arranged to retrieve the spectrum from the detection means, to make measurements on the spectrum, and to assess the blood glucose level of said living body when offered to the window; characterised in that: d. the housing includes a first conductive ring located within the space to produce a first resonance and a second conductive ring located within the space to produce a second resonance; e. the first conductive ring is located within the space such that the first resonance will experience a perturbation by said living body in proximity or contact with the window and the characteristics of the first resonance depend on the blood glucose level of the blood in the tissue in that part of the living body that is adjacent to the window; and f. the second conductive ring is located within the space further from the window than the first conductive ring such that the second resonance will experience a perturbation to a degree negligible relative to the perturbation experienced by the first resonance by said living body in proximity or contact with the window. a microwave resonator having a resonant response to input microwaves and designed such that said response will experience a perturbation by the 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; wherein: a. the resonator comprises a housing defining a space with a microwave-transparent window to which said living body can be offered, the housing including a microwave input to the space and a microwave output from the space; b. the detection means are configured for recording a power versus frequency spectrum of microwaves that have passed through the resonator; c. the monitor further comprises computing means arranged to retrieve the spectrum from the detection means, to make measurements on the spectrum, and to assess the blood glucose level of said living body when offered to the window; characterised in that: d. the housing includes a first conductive ring located within the space to produce a first resonance and a second conductive ring located within the space to produce a second resonance; e. the first conductive ring is located within the space such that the first resonance will experience a perturbation by said living body in proximity or contact with the window and the characteristics of the first resonance depend on the blood glucose level of the blood in the tissue in that part of the living body that is adjacent to the window; and f. the second conductive ring is located within the space further from the window than the first conductive ring such that the second resonance will experience a perturbation to a degree negligible relative to the perturbation experienced by the first resonance by said living body in proximity or contact with the window. a. the resonator comprises a housing defining a space with a microwave-transparent window to which said living body can be offered, the housing including a microwave input to the space and a microwave output from the space; b. the detection means are configured for recording a power versus frequency spectrum of microwaves that have passed through the resonator; c. the monitor further comprises computing means arranged to retrieve the spectrum from the detection means, to make measurements on the spectrum, and to assess the blood glucose level of said living body when offered to the window; d. the housing includes a first conductive ring located within the space to produce a first resonance and a second conductive ring located within the space to produce a second resonance; e. the first conductive ring is located within the space such that the first resonance will experience a perturbation by said living body in proximity or contact with the window and the characteristics of the first resonance depend on the blood glucose level of the blood in the tissue in that part of the living body that is adjacent to the window; and f. the second conductive ring is located within the space further from the window than the first conductive ring such that the second resonance will experience a perturbation to a degree negligible relative to the perturbation experienced by the first resonance by said living body in proximity or contact with the window.