Innovative uses of technology to support teacher learning are emerging through professional learning communities that leverage social networking technologies. The value of an online community for educators lies in the rich and open exchange of ideas, experiences, and resources where educators feel both respected and supported. Yet among the most difficult challenges faced by online communities is fostering and sustaining knowledge sharing and trust. The purpose of this qualitative case study was to examine in-depth successful online learning communities for K-12 educators to better understand ways in which knowledge sharing and trust are cultivated and supported. Cross-case findings indicate knowledge sharing and trust were cultivated and sustained through a clear purpose and common identity, multiple options and opportunities for social learning, the active involvement of an experienced and credible moderator, as well as modeling and enforcement of appropriate online behavior. (Contains 2 tables.)