This document gives an overview of the Earth Observing System (EOS) Microwave Limb Sounder (MLS) experiment. It is intended to provide general information for a wide range of readers including MLS team members, additional scientists who may be using MLS data in their research, and programmatic officials. It also provides introductory and supporting information for the other EOS MLS Data Processing Algorithm Theoretical Basis Documents [Filipiak et al., 2004; Jarnot, et al., 2004a; Jiang, 2004; Livesey and Snyder, 2004; Read et al., 2004a; Schwartz et al., 2004; Wu and Jiang, 2004a]. These documents, and other information on MLS, are available at the MLS web site http://mls.jpl.nasa.gov.