This paper demonstrates a non-invasive method to determine the hydration level of human skin by measuring the reflectance of W-band (75-110 GHz) and Ka-band (26-40 GHz) radiation. Ka-band provides higher hydration accuracy (<1%) and greater depth of penetration (> 1 mm), thereby allowing access to the important dermis layer of skin. W-band provides less depth of penetration but finer spatial resolution (~2 mm). Both the hydration sensing concept and experimental results are presented here. The goal is to make a human hydration sensor that is 1% accurate or better, operable by mechanically scanning, and fast enough to measure large areas of the human body in seconds.