Ignoring the informal, non-canonical nature of knowledge sharing, including people's motivation, ability and opportunity to share knowledge, is one of the key causes of resistance to use knowledge-sharing tools. In order to improve knowledge sharing supported by information technology (IT), tools need to be embedded in the social networks of which it is part. This has implications for our knowledge on the design requirements of such socially embedded IT. The paper reviews tools that are designed for the purpose to foster social capital. We will then discuss what is needed for an IS design theory related to knowledge communities and how such a theory could incorporate social capital theory. © 2006 JIT Palgrave Macmillan Ltd. All rights reserved.