In order to meet the challenging thermal analysis needs of Space Station Freedom and other proposed Space Exploration Initiative missions, a new generation of thermal analysis tools, utilizing the latest advances in user-interface design, software engineering approaches, graphics capability, CPU speed, and networking technology will be needed. NASA Johnson Space Center has commissioned the development of the Thermal Synthesizer System (TSS) to meet these difficult analysis requirements. TSS is being developed by Lockheed Missiles and Space Company, Lockheed Engineering Sciences Company, and NASA Johnson Space Center. The system consists of integrated applications to perform: surface and solid modeling, radiation coupling, orbital analysis, characterization of the thermal radiation heating environment for a spacecraft in orbit or on a planet surface, conduction network generation, two-phase flow modeling and analysis, temperature predictions, and general purpose post-processing. While TSS is tailored for spacecraft thermal analysis, it may be used for any general purpose thermal analysis as well.The TSS user interface allows simultaneous menu and command line input. The command language interpreter is fully programmable to allow the functions of TSS to be combined in more powerful ways, for example, automated optimization of desired thermal design parameters. The user interface has successfully integrated the requirements of a rapidly learned, intuitive, menu driven interface with powerful analysis algorithms into a system that can be fully exploited by the expert engineer.This paper presents the motivation for a TSS and its primary functional and design requirements, its basic design philosophy and software engineering approach, and an overview of the main features of each of the applications within TSS.