While evidence of the exceedingly important role of technology in organizational life is commonplace, academics have not fully captured the influence of Information and Communication Technology (ICT) on crisis response. A substantive body of knowledge on technology and crisis response already exists and keeps developing. Extensive research is on track to highlight how technology helps to prepare to crisis response and develop service recovery plans. However, some aspects of crisis response remain unknown. Among all the facets of crisis response that have been under investigation for some years, improvisation still challenges academics as a core component of crisis response. In spite of numerous insights on improvisation as a cognitive process and an organizational phenomenon, the question of how improvisers do interact together while improvising remains partly unanswered. As a result, literature falls short of details on whether crisis responders can rely on technology to interact when they have to improvise collectively. This dissertation therefore brings into focus ICT support to organizational improvisation in crisis response in two steps: We first address this question from a general standpoint by reviewing literature. We then propose an in depth and contextualized analysis of the use of a restricted set of technologies – emails, faxes, the Internet, phones - during the organizational crisis provoked by the 2003 French heat wave. Our findings offer a nuanced view of ICT support to organizational improvisation in crisis response. Our theoretical investigation suggests that ICTs, in a large sense, allow crisis responders to improvise collectively. It reports ICT properties - graphical representation, modularity, calculation, many-to-many communication, data centralization and virtuality – that promote the settling of appropriate conditions for interaction during organizational improvisation in crisis response. In the empirical work, we provide a more integrative picture of ICT support to organizational improvisation in crisis response by retrospectively observing crisis responders’ interactions during the 2003 French heat wave. Our empirical findings suggest that improvisation enables crisis responders to cope with organizational emptiness that burdens crisis response. However, crisis responders’ participation in organizational improvisation depends on their communicative genres. During the 2003 French heat wave crisis, administrative actors who had developed what we call a “dispassionate” communicative genre in relation to their email use, barely participated in organizational improvisation. Conversely, improvisers mainly communicated in what we call a “fervent” communicative genre. Therefore, our findings reveal that the ICT support to organizational improvisation in crisis response is mediated by the communication practices and strategies that groups of crisis responders develop around ICT tools.