Body composition and tissue hydration are critical parameters that play an essential role in each person’s health status and disease risk. Current techniques for accurately and quantitatively measuring these features either lack sensitivity or are difficult and expensive to operate. Here, I present a new method for evaluating body composition using Diffuse Optical Spectroscopy. I also introduce a Tissue Hydration Index (THI), a method to assess the relative water content or hydration status of a person’s tissue. THI is developed using DOSI technology and a simplified approach for deriving the index using time-independent spectroscopy is proposed and validated.