Abstract: In the safety- And time-critical context of pre-hospital medical care, usability is a major challenge that needs to be addressed, because interaction problems or errors may have serious consequences for patients as well as employees of Emergency Medical Services (EMS). In this regard, a user-centered human-computer interface is a crucial factor for efficient usage as well as user acceptance. Well-established design principles can serve as directives throughout the analysis, design, implementation, and evaluation of applications. For standard PCbased applications the ISO standard 9241-110 serves as a practical guide since many years. However, new types of devices, especially mobile systems impose new challenges and ways of interpreting usability standards and guidelines. In this paper, we apply the ISO-Standard 9241-110 dialogue principles to mobile applications in EMS, considering the challenges of designing a system suitable for supporting users in both regular day-to-day services as well as rare Mass Casualty Incidents (MCIs).