This volume presents the work of trailblazing researchers and developers of electronic communities for professional learning. It illuminates the essential work behind the scenes in building successful online communities and scaffolding site interactions, including content selection, creation and management, administrative structures, tools and interactive functionalities, the facilitation of discourse and emergent sub-communities, and the development of online leadership. While each of the authors is knowledgeable about site design, new technologies, and collaborative Web tools, their work is deeply influenced and informed by scholarship which has unfolded over the last three decades about how professional learning takes place, particularly for math and science educators.