This book constitutes the refereed proceedings of the Second International Conference on Online Communities and Social Computing, OCSC 2007, held in Beijing, China in July 2007 in the framework of the 12th International Conference on Human-Computer Interaction, HCII 2007 with 8 other thematically similar conferences. The 56 revised papers presented were carefully reviewed and selected from numerous submissions. The papers accepted for presentation thoroughly cover the thematic area of online communities and social computing, addressing the following major topics: designing and developing on-line communities, as well as knowledge, collaboration, learning and local on-line communities.