Mass casualty incidents confront first responders of emergency medical services and incident commanders with a large number of casualties as well as the need for coordination of arriving rescue personnel. Both have to manage processes ad hoc and immediately after arriving at the incident area. While first responders must carry out procedures that deviate from routine, incident commanders have to solve complex management tasks. Meanwhile, they have to deal with the problem, that work already takes place while building up management structures. This contribution discusses mental models and situation awareness aspects of first responders and incident commanders. Based on challenges identified, implications for designing and developing computer-based systems for MCIs are explained and demonstrated exemplarily. Computer systems for MCIs must be well-designed for the dynamic context and processes of MCIs as well as for the mental models and necessary situation awareness of the engaged professionals. Only then, they can overcome being documentation or administration tools by becoming impactful cognitive artifacts, that support decision-making and offer guidance and assistance in a demanding mission-critical field.