Twitter is a micro-blogging service on the Web, where people can enter short messages, which then become visible to other users of the service. While the topics of these messages varies, there are a lot of messages where the users express their opinions about com- panies or products. Since the twitter service is very popular, the messages form a rich source of information for companies. They can learn with the help of data mining and sentiment analysis tech- niques, how their customers like their products or what is the gen- eral perception of the company. There is however a great obstacle for analyzing the data directly: as the company names are often ambiguous, one needs first to identify, which messages are related to the company. In this paper we address this question. We present various techniques to classify tweet messages, whether they are related to a given company or not, for example, whether a mes- sage containing the keyword “apple” is about the company Apple Inc.. We present simple techniques, which make use of company profiles, which we created semi-automatically from external Web sources. Our advanced techniques take ambiguity estimations into account and also automatically extend the company profiles from the twitter stream itself. We demonstrate the effectiveness of our methods through an extensive set of experiments.