This thesis investigates a decision aid to support tasks where there is an initial sighting of an object and then the decision maker has to guide some vehicles, either to reestablish contact with the lost object or to stay clear of it. As such, the tasks are analogous to naval search and evasion tasks. The decision aid helps a user to keep track of where enemy units can be by visualizing constraints on the units’ movement. Six studies are included. The first investigates how commanders judge the threat in a tactical situation and what they do to control that threat. The results point to that analysis of the enemy is a difficult task and has the potential to be improved by a support system. The second study investigates how commanders analyze the enemy. The results suggest that the uncertainties regarding the location and behavior of the enemy are central problems for the commander. It further describes a strategy that the participants use to cope with those problems, and the strategy is used as basis for the proposed decision aid. The remaining studies are experiments in which the support system is evaluated. A search task and an evasion task are used, and participants with and without experience from the tasks participate. The results suggest that the decision aid increased performance in both tasks and that both university students and navy officers gained from using it. Further, when using the visualization novice participants performed on the same level as the experienced. It was easy to learn how to use the visualization, and transfer to a situation without it depended on the complexity of its outline. The findings have practical application for designers of command and control support systems as they indirectly suggest that people have problems in inferring the whereabouts of contacts of which they only have fragmentary information. Thus, including the proposed decision aid in the design of such systems may have positive impact on overall system performance.