These days, Web 2.0 and Social Media are terms used everywhere by communications professionals and students. Yet, as this entire field is relatively new compared to the "old media", scientific research about it is still scarce. In contrast to books that rather serve as instructional manuals of how to use Twitter right, this study focuses on what Twitter should be used for, and how it is currently being used in professional communications. In this thesis, the use of Twitter by the American car industry is analyzed in relation to three different fields of corporate communications: Marketing, Public Relations and Corporate Branding. Within the study, the Social Media landscape is classified and Twitter is brought into line with it as a micro-blogging medium consisting of Tweets. A qualitative analysis of over 1.800 of those Tweets show which of the three corporate communications fields is served most by Twitter. The results also describe implications regarding language use and interaction with other Twitter users.