This thesis adopts the use of various simulation-based training exercises as research platforms for assessing the role of environmental, task, and team uncertainty on critical and major incident decision making. It illustrates that experienced uncertainty in these incidents has a significant detrimental “derailing” effect on strategic decision making. Task uncertainty occurs within critical incidents when decision makers face forced choice binary decisions with equally unattractive outcomes. Team uncertainty occurs due to the large, heterogeneous nature of any emergency response organization, often involving individuals from an array of agencies. The studies showed that these types of uncertainty induced derailments from preferred strategies or courses of action. These derailments were manifest as a variety of implementation failures, including omissions or temporary deferral of crucial actions, a failure to share crucial information with other emergency responders, and failing to coordinate or synchronize actions with those others involved in incident management. These findings are in accordance with the widely adopted definition of uncertainty in naturalistic decision making as “a sense of doubt that blocks or delays action” (Lipshitz & Strauss, 1997, p. 150). Moreover, they contribute to the existing literature by illustrating that errors omission, as opposed to actively committed mistakes, are a prevalent occurrence that may impede “optimal” incident management strategies in various types of high risk, uncertain critical incidents.