This dissertation is a qualitative study of occupant behavior in response to the 2001 World Trade Center (WTC) disaster. Through analyses of transcripts from 245 face-to-face interviews with survivors from both WTC towers, collected by Project HEED, I investigate the pre-evacuation period in what became the largest full-scale building evacuation in history. The objectives of this study are to understand the types of actions performed before occupants began evacuation via stairs and elevators and why those actions were taken to improve techniques used and to address inadequacies in evacuation modeling tools. Drawing on social psychological theories of human action in normal times, collective behavior and sensemaking during normative crises, and emergency decision-making and protective action in response to hazards and disasters, and research on decision-making under uncertainty (including the importance of satisficing rationality), I examine how interviewees made sense of environmental cues, interpreted the situation and danger to themselves and others, and decided upon pre-evacuation actions, either to seek confirmation or achieve protection. Subsequently, I developed a predictive conceptual model of pre-evacuation actions by identifying the linkages between occupant- and situationally-based factors and the actions performed. I argue that occupant pre-evacuation behavior in the WTC disaster can be conceptually modeled by understanding both the disaster environment and the meanings individuals assigned to that environment. On 9/11, occupants consistently developed new social norms and lines of action based upon the meanings that occupants assigned to the situation, including perceptions of risk, familiarity with the building and others in the building, and responsibility for others. These meanings were dependent upon the receipt of environmental cues as well as on pre-existing norms, experiences, training, and social roles. My thesis contributes to empirical studies of decision-making during disasters and building fires, emergency planning and management, and the field of computer evacuation modeling by examining the emergency decision-making process and the factors that influenced each stage of this process during the most deadly terrorist attack in U.S. history.