Whenever people talk together they establish and sustain a little world of shared attention and involvement, a “communion of mutual engagement,” as Goffman (1957) described it. As such participation, or better co-participation, in an activity cannot be reduced to mere physical co-presence. Rather, participation (or non-participation) in talk is organized moment-by-moment, incarnately in and through that talk. While this may seem obvious, it raises a series of important questions about the organization of human conduct. Specifically, what are the practices of speaking, listening, gesturing, posturing and so on by which persons constitute themselves and co-present others as participants in social interaction and participants of a particular kind? What forms of participation are possible and how are these realized in the particular activities with which they are associated? In this chapter I trace ideas about this fundamental aspect of human interaction from an initial conception of the “speech circuit” through mid-century notions of “channel” and “phatic communion” to Goffman's innovative rethinking of the issue in terms of “mutual monitoring possibilities” and his trail-blazing analysis in the late essay “Footing”. I conclude with a discussion of current research in conversation analysis, linguistic anthropology and pragmatics that investigates the complex structures and practices of participation that characterize human social interaction.