EMS workers navigate an environment filled with uncertainty, high stakes, time pressure, complexity, and, at times, conditions that pose risk to their personal safety all while managing complex patient care tasks. They make life and death decisions under these extreme conditions. An EMS worker must diagnose injuries and diseases, assess risks, prioritize treatment options, and manage the social environment. The process of decision-making is central to all of these tasks. Therefore, a detailed understanding of how expert EMS workers make decisions can be invaluable for designing systems to support their work as well as developing new expert EMS workers as efficiently as possible.