This volume collects the papers selected for presentation at the Third Inter- tional Conference on Metadata and Semantic Research (MTSR 2009), held in Milan at the University of Milano-Bicocca (October 1-2, 2009). Metadataandsemanticresearchistodayagrowingcomplexsetofconceptual, theoretical, methodological, and technological frameworks, o?ering innovative computational solutions in the design and development of computer-based s- tems.Fromthis perspective,researchersworkinginthisareamusttackleabroad range of issues on methods, results, and solutions coming from di?erent classic areas of this discipline. The conference has been designed as a forum allowing researchers to present and discuss specialized results as general contributions to the ?eld. In order to give a novelperspective in which both theoreticaland application aspects of metadata research contribute in the growth of the area, this book mirrors the structure of the conference, grouping the papers into three main categories: (1) Theoretical Research: Results and Proposals; (2) Applications: Case Studies and Proposals; (3) Special Track: Metadata and Semantics for Agriculture,FoodandEnvironment.Thebookcontains31fullpapers(10forthe ? rstcategory,10forthesecondand12forthethird),selectedfromapreliminary initial set of about 70 submissions. Many people contributed to the success of the conference and the creation of this volume, from the initial idea to its implementation. Our ?rst ackno- edgement is to the members of the Steering Commitee, GeorgeBokosand David Raitt. We would also like to thank all Program Committee members and - viewers for their collaboration. Special thanks to Carlo Batini, on behalf of the DepartmentofComputerScience,SystemsandCommunicationoftheUniversity of Milan-Bicocca, who kindly hosted our conference.