Abstract: An apparatus and method for non-invasive determination of hydration, hydration state, total body water, or water concentration by quantitative spectroscopy. The system includes subsystems optimized to contend with the complexities of the tissue spectroscopy, high signal-to-noise ratio and photometric accuracy requirements, tissue sampling errors, calibration maintenance, and calibration transfer. The subsystems include an illumination subsystem, a tissue sampling subsystem, a spectrometer subsystem, a data acquisition subsystem, a computing subsystem, and a calibration subsystem.