Suppose you're a member of a few development teams, working with people who are geographically dispersed. You're using distributed groupware to work with your team mates. How do you decide when to work on a project and when to ignore requests to work on a project, when there are enough users on the groupware system to bother using it, who is available to answer a question, and which applications should get the most real estate on your screen? To help answer these questions, distributed groupware systems must indicate something about the social world they represent-who is on the system and what they are doing. User interfaces for groupware (or computer supported cooperative work (CSCW) applications) must therefore convey social information. It's energizing to know, for example, that your team mates are busy working away on a project. And it's nice to know when your friends or colleagues are available on a chat system. You might not need to know the semantics of the messages or documents involved, just that some activity is occurring. This is true for systems used by work groups as well as those used by an organization or a community of users. We think such social indicators should be a standard part of the CSCW user interface. On the basis of social psychology theory, we believe that a class of social indicator, which we call social activity indicators, is a simple, powerful way to improve user-interface functionality. Furthermore, social activity indicators are easy to build.