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