Chilled Mirror Hygrometers are used in Standards and Metrology labs as well as in industrial applications where precise and repeatable humidity measurement and control is required. The inherent accuracy and long term stability provides many advantages over other types of humidity measurement technologies. Since chilled mirrors fundamentally measure the dew or frost point temperature directly by controlling a reflective surface to equilibrium between dew/frost formation and evaporation and precisely measuring the temperature of the mirror at this point, the methodology has been validated by standards labs worldwide. General Easten’s chilled mirrors are capable of measuring dew/frost points (Td) from -80°C to +85°C with an accuracy of +/- 0.15°C. An introduction of the principles and theory of operation of chilled mirrors follows.