We are developing a Web-based tool called WebGuide to mediate and structure collaborative learning. This software uses an innovative mechanism to define a flexible system of perspectives on a shared knowledge construction space. WebGuide provides an electronic and persistent workspace for individuals and teams to develop and share distinctive points of view on a topic. We are designing the software and associated usage practices by trying it out in a middle school classroom and an advanced graduate seminar. Our experience in these use situations has raised a range of questions concerning theoretical and practical issues, which are driving our research. This paper is a reflection on what we are learning collaboratively about how software artifacts can mediate learning and shared cognition.