A new process is described that assists engineers in making complex decisions during the design process. The Interactive Design Selection Process (iDSP) allows designers to collect design information, compare candidate designs, build user preferences, and ultimately zero in on the best designs. In contrast to traditional black-box optimization approaches, the iDSP involves the designer in each and every step of the process. This involvement gives designers more confidence in the results and allows them to find designs that satisfy all stake-holders. A prototype of the iDSP was implemented by combining an advanced visualization tool with a genetic optimizer using a commercially available process integration environment. A Lunar exploration mission design problem was solved using the guided search technique augmented by design space visualization.