The promise of the Semantic Web is to move from a Web of data to a Web of meaning and distributed services. This vision of the Web has attracted - searchersfromdi?erent horizonswith the aims of de?ning newarchitecturesand languages necessary to make it possible, and of developing the ?rst applications of these concepts. This book contains the articles selected for publication and presentation at the workshop "Principles and Practice of Semantic Web Reasoning" PPSWR 2005, together with three invited talks. Three major aspects of Semantic Web research are represented in this selection: architecture issues, language issues, and reasoning methods. These advances are investigated in the context of new design principles and challenging applications. ThePPSWR2005workshopwaspartoftheDagstuhlseminarontheSem- tic Web organizedby F. Bry (Univ. Munchen, Germany),F. Fages (INRIA Roc- .. quencourt, France), M. Marchiori (MIT, Cambridge, USA) and H.-J. Ohlbach (Univ. Munchen, Germany),held in Dagstuhl, Germany,11-16September 2005...It was supported by the European Network of Excellence REWERSE (Reas- ing on the Web with Rules and Semantics, http://rewerse.net). This four-year project includes 27 European research and development organizations, and is intended to bolster Europe's expertise in Web reasoning systems and appli- tions.Itconsistsofeightmainworkinggroups:"RuleMarkupLanguage","Policy Language, Enforcement, Composition","Composition and Typing","Reasoning- Aware Querying","Evolution","Time and Location","Adding Semantics to the Bioinformatics Web", and "Personalized Information Systems". The papers in this volume re?ect most of the topics investigated in REWERSE; one third of them come from outside REWERSE.