"The decision-making expertise of battle commanders" Joint Task Force commanders and their subordinates are highly experienced officers, particularly with respect to combat operations. Command decision making is a critical skill for these officers, but it is a skill that is difficult to define, assess, and train. Opportunities for real-life experiences are rare. Researchers in Naturalistic Decision Making are often concerned with defining and evaluating expertise in complex domains where there are no operationalized expertise levels, no single correct answers to problems, and where the observation and measurement of real-world expert performance is difficult. The research effort described in this paper builds on a mental model theory of expert decision making to design an experiment in which expertise is assessed in an extremely complex and demanding domain: command decision making in tactical warfare. The experiment demonstrates that command decision making expertise can be recognized by domain experts, differences in the command decision making expertise of individuals can be identified, and observers who are not domain experts can recognize the expert behaviors predicted by a mental model theory about the nature of expertise. The authors recommend that the indicators of decision-making expertise identified in this research should be used to guide assessment of command proficiency and to target the training of specific command skills.