This article describes a speech-based user interface to a wide range of entertainment, navigation and communication applications in mobile environments by means of human-machine dialogues. The system has been developed in the framework of the EU-project SENECA. It uses noise reduction, speech recognition, and dialogue processing techniques. One interesting aspect relies in the fact that low speech recognition confidence and word-level ambiguities are compensated by engaging flexible clarification dialogues with the user. The SENECA system demonstrator has been evaluated by means of user tests. With speech input, road safety, especially for complex tasks is significantly improved. Compared to manual input, the feeling of being distracted from driving is less important with speech.