This document provides the physics contained in the signals measured by the Microwave Limb Sounder (MLS) experiment on the National Aeronautical and Space Administration (NASA) Earth Observing System (EOS) Aura (formerly CHEM) mission scheduled to launch in June 2004. The mathematical function solved herein is called the forward model and describes the relationship between measured signals and atmospheric composition. "Signal" for the purposes of this document is atmospheric spectral intensity in brightness (K). The conversion between brightness and the basic engineering measurements made by the instrument hardware itself is described in Jarnot, et al, 2004.