People often make decisions without gathering or processing all possible information. Time pressure, high information acquisition costs, too much information, and incomplete information in the environment, all contribute to this reality. This thesis is motivated by the need for decision support tools for these heuristic decision makers. The foundational contributions will be a new understanding of heuristic decision making with incomplete information and how to use this knowledge to design a heuristic decision support tool. Decision support tools are built upon understanding fundamental human decision making tendencies. The open question regarding how distributions of incomplete information affect decision making performance (both heuristic and normative) must be answered before information acquisition methods for supporting heuristic decision making strategies can be developed. This thesis intends to contribute to the basic research of decision making with incomplete information in order to contribute to the applied research of designing support tools for heuristic decision makers. First, this thesis will develop a simulation engine to simulate decision making with incomplete information in order to identify plausible methods of information restriction and guidance for decision support. Then, a human-subjects study will examine decision making with time pressure and incomplete information to search for evidence of the potential effectiveness of the methods. Lastly, a decision support tool will be constructed based on the information restriction and guidance methods and tested in a simulated unmanned aerial vehicle operations task.