Social media have established themselves as a relevant field of research in various disciplines, among them public relations and communication studies. While the focus during the past few years has been on the advantages that these offer for organisations, for example customer relationships and sales, this thesis argued that social media also pose an immanent risk for corporate reputation, which manifests itself in so-called “social media crises”. During a social media crisis, online users and non-governmental organisations attack business pages on social networking sites and damage their reputation. Often, negative emotions play a decisive role. This research looked at three case studies of multinational for-profit organisations that have experienced a social media crisis. The aim was to identify how the social networking sites Facebook and Twitter contributed to the development of reputational crises, how this impacted the affected companies and what response strategies were used. To take the growing influences of external parties on corporate reputation into account, an interdisciplinary approach was employed, using a methodology of psychology and sociology. A multiple-case study method was used to allow for explanation building across cases. Qualitative and quantitative data was triangulated through the curating tool Storify (http://storify.com/LarissaOtt1). In order to assure the reliability, a theoretical framework of signaling theory and Situational Crisis Communication Theory was applied. While the former was used to analyse reputation management, the latter was employed to identify crisis response strategies, but also to evaluate how successful this traditional approach was in the social media. The data analysis found that due to the heightened public scrutiny during a social media crisis, adhering to transparency and finding an authentic, coherent voice throughout traditional and social media was a crucial factor. Overall, the influence of the traditional media was still visible. Although they did not initiate the crisis, they added to its severity. In regard to crisis response strategies, deleting negative comments proved to be counterproductive. While dialogue has often been emphasised in literature on public relations online, this research found that companies seldom employ dialogue, but focus on persuasion and negotiation. Moreover, dialogue is not always advised for, especially if the discussion gets too heated and strong negative emotions are involved. Consequently, companies have to adapt their strategy according to the situation when interacting with individuals online. During the crisis situation, accommodative strategies seemed to be more effective than deny or diminish response strategies. In general, building a reputational reservoir before a crisis, possibly through corporate social responsibility, and providing an open forum for discussion that also allows supporters of the company to speak up was identified as another factor for success. The results of this study contribute to an understanding of the nature and dynamic of social media crises and offer helpful strategies for public relations practitioners. Further need for research was identified to adapt the theoretical concepts that are available in the area of crisis communication for social media crises and to keep up with the fast-evolving world of social networking sites.