The Moderate Resolution Imaging Spectroradiometer (MODIS) instrument is the key facility instrument on board the NASA-GSFC “Mission-to-Planet Earth” EOS-AM spacecraft. This instrument is designed to study Earth system processes and includes 36 spectral bands for study of oceanographic, atmospheric, and land surface phenomenon. Launch of this 705 km high, polar orbiting platform, mounted atop an Atlas IIAS, is scheduled for June, 1998 from Vandenberg AEB, California. The MODIS Protoflight instrument has been delivered to Lockheed Martin Marietta’s Valley Forge facility for spacecraft integration and testing. The primary structure of the MODIS instrument is the “mainframe” and this paper discusses the geometrical design, material selection and processes, static and dynamic analyses and environmental testing required to ensure spaceflight reliability. Comprehensive studies of candidate materials led to the selection of beryllium made by the “Hot Isostatic Process”.