This special issue of TLDKS contains two kinds of papers. First, it contains a sel- th tion of the best papers from the 11 International Conference on Data Warehousing and Knowledge Discovery (DaWaK 2009), which was held from August 31 to S- tember 2, 2009 in Linz, Austria. Second, it contains a special section of papers on a particularly challenging domain in information retrieval, namely patent retrieval. Over the last decade, the International Conference on Data Warehousing and Knowledge Discovery (DaWaK) has established itself as one of the most important international scientific events within data warehousing and knowledge discovery. DaWaK brings together a wide range of researchers and practitioners working on these topics. The DaWaK conference series thus serves as a leading forum for discu- th ing novel research results and experiences within the field. The 11 International Conference on Data Warehousing and Knowledge Discovery (DaWaK 2009) cont- ued the tradition by disseminating and discussing innovative models, methods, al- rithms, and solutions to the challenges faced by data warehousing and knowledge discovery technologies.