The overall aim of our research group is to enhance patients’ safety. The present research project focused on the importance of human factors in medical emergencies. Specifically, the aim of the was to test the hypothesis that particularities of medical emergency-driven teams, i.e. continuous enlargement, integration of newcomers and change in leadership, represent a special challenge that affects the technical performance and is a source of specific human error.Modern health-care is a team endeavour. Health-care in emergency situations is delivered by medical emergency driven teams, i.e. teams that form ad-hoc for the specific emergency and disintegrate after completion of the task. These teams are characterised by interdisciplinarity, by team members arriving at different times, and by changes in leadership as more senior and experienced members usually arrive late in the process of team-building. 60% to 80% of errors in the medical field are due to human factors. Most of the mechanisms underlying human error have been studied in individuals and there is only little data on human error in the team context relevant for medical settings. Though there is extensive research with regard to group performance in general, the particularities of ad-hoc teams has not been studied. The research was conducted in a high-fidelity patient simulator at the University of Basel. Participants include nurses and physicians. Data analysis was performed using videotapes recorded during simulations. A particular strength of our methodology is micro-coding with regard to behaviour and communication, as developed by our research group. The key findings of our research project were as follows: 1) compared to teams being present right from the start of a medical emergency teams forming ad-hoc during the emergency provided less patients’ support; 2) unnecessary interruptions of cardiopulmonary resuscitation occur frequently but are not recalled; 3) substantial delays in crucial measures of resuscitation occur in ad-hoc forming interdisciplinary teams of first responders in intensive care; 4) direct leadership of the nurse first present at an emergency and swift taking over of the lead by the first physician arriving at the scene is associated with an enhanced team performance; 5) higher communication density per se is not related to a better group performance; 6) during a medical emergency, a substantial proportion of relevant information is transmitted inaccurately to the incoming physician in interdisciplinary ad-hoc forming teams.