A hydration and composition measurement device and technique that in one embodiment includes a microcontroller; a phase lock loop frequency synthesizer controlled by the microcontroller to generate radio frequencies; a cavity for use in resonating at a set of frequencies; a radio frequency power detector for measuring perturbed cavity resonance; and an analyzer. The hydration and composition measurement device may further include a circulator, which receives frequencies from the phase lock loop frequency synthesizer, and an antenna connected to the circulator and cavity. The cavity may be defined by a metallic material or members, and the cavity includes a hole configured to provide access of tissue to be tested to the cavity.