The LNCS journal Transactions on Large-Scale Data- and Knowledge-Centered Systems focuses on data management, knowledge discovery, and knowledge processing, which are core and hot topics in computer science. Since the 1990s, the Internet has become the main driving force behind application development in all domains. An increase in the demand for resource sharing across different sites connected through networks has led to an evolution of data- and knowledge-management systems from centralized systems to decentralized systems enabling large-scale distributed applications providing high scalability. Current decentralized systems still focus on data and knowledge as their main resource. Feasibility of these systems relies basically on P2P (peer-to-peer) techniques and the support of agent systems with scaling and decentralized control. Synergy between grids, P2P systems, and agent technologies is the key to data- and knowledge-centered systems in large-scale environments. This, the sixth issue of Transactions on Large-Scale Data- and Knowledge-Centered Systems, contains eight extended and revised versions of papers selected from those presented at DEXA 2011. Topics covered include skyline queries, probabilistic logics and reasoning, theory of conceptual modeling, prediction in networks of moving objects, validation of XML integrity constraints, management of loosely structured multi-dimensional data, data discovery in the presence of annotations, and quality ranking for Web articles.