Technologies that electronically mediate conversation, such as text-based chat or desktop video conferencing, draw on theories of human−human interaction to make predictions about the effects of design decisions. This lecture reviews the theory that has been most influential in this area: Clark's theory of language use. The key concept in Clark's theory is that of common ground. Language is viewed as a collaborative activity that uses existing common ground to develop further common ground and, hence, to communicate efficiently. The theory (a) defines different kinds of common ground, (b) formalizes the notion of collaborative activity as a "joint action," and (c) describes the processes by which common ground is developed through joint action. Chapter 1 explains why a purely cognitive model of communication is not enough and what is meant by the phrase "collaborative activity." Chapter 2 introduces the idea of common ground and how it is used in language through an example of two people conversing over a video link. Chapter 3 indicates where the interested reader can find out about the antecedents to Clark's theory. Chapter 4 sets out the fundamental concepts in Clark's theory. Chapter 5 uses five published case studies of electronically mediated communication to illustrate the value of the theory. These include studies of a computer-supported meeting room (Cognoter), a video tunnel that supports gaze awareness, video conferencing in medical consultation, and text chat. Human-centered informatics (HCI) started as the study of an individual interacting with a computer. Very quickly, it became clear that digital devices have enormous potential for communication and the discipline moved on to encompass technologies that electronically mediate human-human interaction, such as text-based chat or video conferencing. The designers of such facilities need answers to questions that depend on a knowledge of how we use language. What communication tasks will benefit from a shared whiteboard? When are text messages better than speech? The theory that informs the design of these artifacts is a theory of human-human interaction (i.e., language use).Previous theories of language use are divided into the cognitive and the social. Most psycholinguistic accounts of language production and comprehension are very cognitive. They are solely concerned with an individual's behavior and the information processing going on in that individual's head. Ethnomethodological and other sociological accounts of language use are, in contrast, social. They concentrate on the structure that is observable in the behavior of groups. Herbert Clark has developed a theory of language use that bridges these two camps. In Clark's theory, individuals have their own individual goals and behavior but also collaborate in such a way that something more emerges when one considers them as a group. To make this step from the individual to the social, the theory defines the notion of a collaborative activity and outlines the processes needed for a collab...