Emergency management and planning often involves multiple domain experts with diverse knowledge backgrounds and responsibilities. Current practices in emergency management and planning have not leveraged the state-of-art technologies in information sharing, synthesis, and analysis. The proposed research will investigate the process of collaborative sensemaking in emergency planning and implement a mapbased online system to support this process. Categories and Subject Descriptors H.5.3. [Information interfaces and presentation (e.g., HCI)] Group and Organization Interfaces: Computer-supported cooperative work General Terms: Measurement, Design, Experimentation