The importance of overhearing, and other ways of monitoring communicative behaviour not explicitly directed at oneself, has been illustrated in numerous ethnographic studies of computer-supported cooperative work. This paper is concerned with a particular form of monitoring. A &&peripheral participant'' is de"ned as someone who has a legitimate interest in monitoring a joint task (being carried out by some &&primary participants'') but who is not actively involved in carrying out the task themselves. The concept is illustrated through "eld studies of telemedical consultation and related to other analyses of overhearing. Two experiments are reported where participatory status was manipulated using a role-play task. Ratings of interpersonal awareness, measures of gaze direction and recall of the conversation all indicate that the task successfully operationalized the distinction between primary and peripheral participation. In addition, the experiment manipulated the visibility of the peripheral participant to a remote primary participant. This was shown to have an e!ect on the remote primary participant's interpersonal awareness of the peripheral participant. Potential mechanisms for this e!ect are considered. It is concluded that peripheral participation is a potentially important form of involvement that needs to be considered when designing and con"guring equipment for videomediated cooperative work.