In this article we focus on the challenges we have encountered in attempting to support the development of an online community of practice for grade 5-12 mathematics and science teachers. Specifically, this project involves the design and evaluation of an electronic knowledge network, the Inquiry Learning Forum (ILF), a web-based professional development system designed to support a community of practice (CoP) of in-service and preservice mathematics and science teachers who are creating, reflecting upon, sharing, and improving inquiry-based pedagogical practices. This research examines the interplay among a variety of variables that characterize the dynamics of building a social network through which participating teachers will seek to share and improve their pedagogical practices. Our research suggests that designing for virtual communities involves balancing and leveraging complex dualities from the "inside" rather than applying some set of design principles from the "outside." This research provides an illuminative case study from which others can more readily identify patterns occurring in their own interventions and navigate the challenges they face more intelligently.