A computer-based design system has been developed for conceptual design of spacecraft active thermal control systems (ATCS). Its purpose is to aid the thermal systems engineer in selection of a working fluid and thermal cycle for a given set of requirements. It allows the user to specify the desired relative importance of several criteria, including system mass, pumping power, and fluid toxicity. The design problem is formulated as a coupled selection-compromise Decision Support Problem, and solved concurrently using DSIDES, a design software package developed at the University of Houston. By using this system, a designer is able to analyze both vapor compression and pumped two-phase thermal cycles, and up to four working fluids simultaneously.A case study based on Space Station Freedom thermal control requirements has been implemented. The fluids considered are Water, R-11, R-114, and Ammonia. Different fluid combinations are selected depending on relative importances of the attributes. R-11 seems to be the best fluid overall, while R-114 is selected when toxicity is emphasized, and Water when power and weight alone are emphasized. It is found that reductions in power system weight penalty result in considerable weight savings for vapor-compression systems, making them more attractive for future spacecraft. The advantage of the present approach lies in the capability to accomodate quantitative technical information as well as environmental and safety considerations in a single model.