A man-machine dialogue system employing an interactive computer system comprising an input device for receiving input from the user. An output device generating output to the user. There is provided an object system which is an information source or store, or a command and control device. The system has a dialogue manager, which orchestrates the dialogue between the object system and user dependent upon a dialogue specification. The specification employs a set of augmented transition networks (ATNs), through which are propagated one or more tokens, the token comprising a set of fields which together define the current state of belief of the dialogue controller, dependent upon the preceding user-system interchanges and information obtained from the object system.