Abstract: Disclosed is a device for transdermally detecting the concentration of one or more substances in a subject's bloodstream. The device includes a signal generation module arranged to generate microwave frequency signals at one or more discrete frequencies. The device also includes a sensor module comprising at least one microwave resonance sensor arranged to make contact with a subject's skin, the at least one microwave resonance sensor arranged to transmit microwave frequency signals generated by the signal generation module into the subject's body. The device also includes a signal processing module connected to the at least one microwave resonance sensor and arranged to: detect a resonance characteristic of the at least one microwave resonance sensor, and process the resonance characteristic to determine the concentration of one or more substances in a subject's bloodstream.