Designing computing systems for frontline firefighting is an open challenge. As of today, little computing support exists for such hazardous environments and designers struggle to build appropriate systems that fit the complex configuration on the frontline. Following Christopher Alexander’s understanding, design is about producing living transformations of existing configurations, it requires a thorough understanding of the situation on-site. Alexander introduces pattern languages as a means to describe existing configurations and to make them accessible for design, to link ethnography and design. This thesis therefore develops a pattern language of firefighters’ activities at the frontline to transform the existing practice into a design space for computing support. Grounded theory, as a qualitative method to identify patterns in empirical data, and action research, as a framework that allows studying the interaction between new technologies and existing practice, solidify the methodology of pattern research and are applied to conduct and analyze workshops with French and German firefighters at professional training facilities. Workshops comprise the observation of existing practice, the active participation in firefighting exercises and the introduction of novel artifacts. Linked up as a pattern language, 16 patterns describe the configuration of frontline firefighting. The patterns detail how firefighters organize the division of roles and tasks, how they deal with information in a dynamic environment, how they form a social binding, improvise, provide safety and prepare their work. While similar individual patterns have been described for firefighting and other high reliability professions, the pattern language, beyond these aspects, provides an integrated perspective on the frontline work; it allows developers to reflect technological concepts and supports the participatory design process of ubiquitous computing systems.