In this chapter we begin by featuring the main characteristics of the human decision process. Then, from these traits, we separate the decision process between diagnosis and look-ahead. We explain why DMSSs are mainly look-ahead machines. We claim that look-ahead is generally performed via heuristic search and “what-if analysis” at different cognitive levels. This leads to a functional definition of DMSSs and to different architectures adapted to heuristic search and moreover, paves the way for an analysis of the decision support tools.