Clark (1996) has identified common ground-information we take to be shared with others-as an indispensable requisite for everything people do with each other where language use or, more definitively, a collaborative use of meaning and understanding is involved. In interactive software development, though, theory and practice have placed little emphasis on the role and importance of this crucial aspect of cognition in user interactions, with the result that user interface and interaction designs are typically impoverished in ways they need not be. In this paper, the basic features of a language use approach to human-computer interaction are outlined, and a range of both noncomputational and computational implications for the design of interactive systems is examined. In particular, human-computer interaction is recast as a genuine instance of language use between the user and the system designer and, in a second layer, as a joint activity in which the system and the user are participants. In this view, principled interaction design is authorship that promotes common ground with the user at all times in all layers through noncomputational and/or computational means. Noncomputational means facilitate the user's development and maintenance of common ground with the author's design through passive and semipassive mechanisms that may also depend on user initiative. Computational means focus on actively maintaining a system-side image of common ground and using this to inform aspects of the system's interaction behavior. Key issues in computing common ground include representing and verifying what the user does and does not know, identifying and using conventions, and solving coordination problems posed by the user.