Abstract. We present the dialogue system of a robot that has been developed to serve as a museum guide. The robot interacts with human visitors in natural language, receiving instructions and providing information about the exhibits. Moreover, being mobile, it physically approaches the exhibits it provides information about. Although the robotic platform contains many modules, including navigation, speech recognition and synthesis, our focus in this paper is the dialogue system, which supports the sessions between humans and the robot, as well as the natural language generation engine, which generates the text to be spoken. Both modules are closely intertwined and depend on an ontology represented in OWL. The robot supports dialogues in both English and Greek.