The TOPEX/POSEIDON spacecraft is scheduled for launch in June 1992 aboard an Ariane 42P vehicle. The TOPEX/POSEIDON mission is a collaborative scientific venture between NASA and CNES. The primary objective of the TOPEX/POSEIDON mission is to perform precise measurements of the ocean surface topography by radar altimetry from a precision, circular orbit. The satellite consists of two major elements, the Multimission Modular Spacecraft bus and the Instrument Module. The IM accommodates seven instruments, the solar array, three communication antennae, and a variety of satellite support equipment. NASA develops five of the instruments, CNES develops the other two.The IM thermal design is a non-optimized design representing a compromise in the accommodation of unique mission requirements. These requirements include: a high radiation environment at 64.45 degrees inclination, an orbit of 1335 km, a periodic sinusoidal yawing about the nadir axis, an outer surface continuity requirement, the splitting of the IM structure in half in order to accommodate integration of the primary instrument at a separate facility, instrument proximity requirements, and stringent temperature rate-of-change and temperature gradient requirements.This paper describes the primary mission objective, the overall spacecraft configuration, the IM requirements and design drivers, the mission environments, the IM design parameters and approach, the IM thermal design, and the analytical temperature predictions versus requirements.