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 subcommunities, and the development of online leadership. While all of the authors are well versed in Web site design, new technologies, and collaborative Web tools, their work is deeply influenced and informed by scholarship that has unfolded over the last three decades about how professional learning takes place, particularly for math and science educators. The communities and authors featured provide different forms of online professional development for university professors, K–12 teachers, administrators, and others. Their insights will be of interest to anyone designing, sustaining, or studying electronic learning environments, regardless of the specific subject matter.