In this paper, two case studies are highlighted that relate to hypothetical decision support in the acquisition of major defence systems through design systems modelling and exploration. With defence acquisition processes becoming more global, the professional experiences of the “detailed systems designer” and the “client specifier / evaluator” are diverging. However, the notion of the client being an informed customer pervades and making right judgements in writing specifications and evaluating tenders relies on reliable design decision modelling and exploration. In providing design decision support to the acquisition organisation, we subscribe to and have applied the paradigm of “decision-based design” using the constructs of the Decision Support Problem Technique (DSPT). The technique offers a holistic systems approach that is flexible, adaptable and able to cope with change.