Significant features of an interactive system that makes the user view it as a user-friendly di alogue system are introduced and discussed. It is argued that it is onl when they are satisfied jointly and independently from dialogue state that they can appreciably affect the degree of system conviviality. It is also claimed that the way that the satisfaction of these features is handled, namely extensionally or as a consequence of a deeper "intelligence" of the system, determines a fundamental division in the approaches of dialogue system d esign. Classical (structural and plan-based) approaches to dialogue are discussed. Then, a new paradigm fo designing and implementing dialogue systems, that of rational dialogue agent, based on rational interaction approach, is introduced. It is shown how this paradigm and approach have led to Artimis, a technology of rational agency, which provides a generic framework to instantiate effective advanced dialogue systems.