This article argues that a distinctive aspect of computer-mediated communication (CMC) is the way it can make communication visible to others in ways that were previously impractical. We propose a theory of communication visibility that recognizes its multidimensional nature: resulting from activities that make communication visible, efforts by actors to see communication, and a sociomaterial context that influences possibilities for visibility. The different dimensions of communication visibility are explored as they relate to possibilities for action with CMC, and the ability of third-parties to view communication between others. Centering communication visibility in the study of CMC compels scholars to ask new questions regarding the interdependence of active, strategic efforts to make communication more or less visible to others, and the ways in which communication is assessed by observers. To facilitate ongoing research we offer an agenda for incorporating communication visibility into the study of contemporary and future forms of CMC.