Volume XII of the Transactions on Rough Sets (TRS) includes articles that are part of a special issue on Rough Set Structuring of Knowledge. These articles are extended versions of papers accepted for presentation at the Rough Set and Knowledge Technology Conference (RSKT 2008) organized in Chengdu, China, in May 2008. In fact, this conference did not take place because of the earthquake that dramatically hit the Chengdu province just before the event. The editors selected some papers accepted for RSKT 2008 and invited their authors to submit extended versions to this issue. The 11 submissions received after this invitation were sent to reviewers and, ?nally, 8 papers were accepted for publication in the special issue. The editors of the special issue are particularly grateful to all the authors of submitted papers, and to the editors of Transactions on Rough Sets,JamesF. Peters and Andrzej Skowron, who provided friendly and valuable assistance in the reviewing process. Special thanks are due to the following reviewers: Mohua Banerjee, Anna Gomolinsk ' a, Ryszard Janicki, Julia Johnson, Maneesh Joshi, Jusheng Mi, Sonajharia Minz, Sushmita Mitra, Tuan Trung Nguyen, Georg Pe- ' ters, Andrzej Skowron, Genlou Sun, Marcin Szczuka, Dominik Sl, ezak, Guoyin Wang, Hai Wang, Marcin Wolski, Haiyi Zhang, Yan Zhao, William Zhu. Their laudable e?orts made possible a careful selection and revision of submitted manuscripts.