In this paper, we demonstrate how field studies, interviews, and low-fidelity prototypes can be used to inform the design of ubiquitous computing systems for firefighters. We describe the artifacts and processes used by firefighters to assess, plan, and communicate during emergency situations, showing how accountability affects these decisions, how their current Incident Command System supports these tasks, and some drawbacks of existing solutions. These factors informed the design of a large electronic display for supporting the incident commander, the person who coordinates the overall response strategy in an emergency. Although our focus was on firefighters, our results are applicable for other aspects of emergency response as well,due to common procedures and training.