The design of user interfaces has lacked a firm theoretical foundation. A solid user interface theory would be particularily helpful in complex systems, such as GIS. Information theory and user modelling are presented here as a promising approach to building such a theory. Their applicability is considered in the context of querying a geographical database. Different theories of information are reviewed and some misunderstandings are corrected. Any communication can be examined on three different levels: syntactic, semantic and pragmatic. The syntactic information of a message is its relative frequency within language without any concern for the meaning of the message. The semantic information content of a message is defined as how many different states of the universe distinguishable in the language it excludes logically. The pragmatic information is the personal meaning of the message to the sender and the receiver. User modelling methodology is presented as a way to handle pragmatic information. Research into user models is reviewed and the potential of GIS applications and further study are investigated.