EOS MLS is a successor to the MLS experiment [Waters et al., 1999; Barath et al., 1993] that formed part of the Upper Atmosphere Research Satellite (UARS), launched in September 1991 [Reber, 1993; Reber et al., 1993]. The instrument is designed to study aspects of the chemistry and dynamics of the atmosphere, from the upper troposphere to the mesopause. The microwave heterodyne technique is employed to observe thermal microwave emission from the Earth's limb in several spectral bands, designed to characterize emission from O2 (used to obtain temperature and pressure information), O3, H2O, ClO, HCl, HNO3, N2O, CO, OH, SO2, BrO, HOCl, HO2, HCN and CH3CN. EOS MLS is one of four instruments on the EOS Aura spacecraft. The launch of the Aura platform is planned for July 2004. Aura will fly in a 98 degree inclined orbit, at a nominal height of 705 km, with a nominal orbital period of 98.9 minutes.