This volume constitutes the selected paqpers of the third international conference on Metadata and Semantic Research, MTSR 2009, held in Milan, Italy, in September/October 2009. In order to give a novel perspective in which both theoretical and application aspects of metadata research contribute in the growth of the area, this book mirrors the structure of the Congress, 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, food and environment. The book contains 32 full papers (10 for the first category, 10 for the second and 12 for the third), selected from a preliminary initial set of about 70 submissions.