may not be well established. by modality text-based spoken dialogue system graphical user interface multi-modal by device telephone-based systems PDA systems in-car systems robot systems desktop/laptop systems native in-browser systems in-virtual machine in-virtual environment robots by style command-based menu-driven natural language speech graffiti by initiative system initiative user initiative mixed initiative Natural dialogue systems "A Natural Dialogue System is a form of dialogue system that tries to improve usability and user satisfaction by imitating human behaviour" (Berg, 2014). It addresses the features of a human-to-human dialogue (e.g. sub dialogues and topic changes) and aims to integrate them into dialogue systems for human-machine interaction. Often, (spoken) dialogue systems require the user to adapt to the system because the system is only able to understand a very limited vocabulary, is not able to react to topic changes, and does not allow the user to influence the dialogue flow. Mixed-initiative is a way to enable the user to have an active part in the dialogue instead of only answering questions. However, the mere existence of mixed-initiative is not sufficient to be classified as a natural dialogue system. Other important aspects include: Adaptivity of the system Support of implicit confirmation Usage of verification questions Possibilities to correct information that has already been given Over-informativeness (give more information than has been asked for) Support negations Understand references by analysing discourse and anaphora Natural language generation to prevent monotonous and recurring prompts Adaptive and situation-aware formulation Social behaviour (greetings, the same level of formality as the user, politeness) Quality of speech recognition and synthesis Although most of these aspects are issues of many different research projects, there is a lack of tools that support the development of dialogue systems addressing these topics. Apart from VoiceXML that focuses on interactive voice response systems and is the basis for many spoken dialogue systems in industry (customer support applications) and AIML that is famous for the A.L.I.C.E. chatbot, none of these integrate linguistic features like dialogue acts or language generation. Therefore, NADIA (a research prototype) gives an idea of how to fill that gap and combines some of the aforementioned aspects like natural language generation, adaptive formulation, and sub dialogues. Performance Some authors measure the dialogue system's performance in terms of the percentage of sentences completely right, by comparing the model of sentences (this measure is called Concept Sentence Accuracy or Sentence Understanding). Applications Dialogue systems can support a