These are the proceedings of the Third International Symposium on Quantum Interaction (QI-2009), held at the German Research Centre for Arti?cial Int- ligence (DFKI), Saarbru ¨cken during March 25-27, 2009. Quantum
theory (QT) is being applied to domains such as arti?cial intelligence, human language, cognition, information retrieval, biology, political science, e- nomics,organizations,andsocialinteraction. After highly
successfulmeetings at Stanford (QI-2007)and Oxford(QI-2008),QI-2009brought together researchers interested in advancing and applying the methods and structures of QT to these and other domains outside of quantum physics: -
Advancement of theory and experimentation for applying quantum theory to non-quantum domains - Use of quantum algorithms to address, or to more e?ciently solve, problems in non-quantum domains (including contrasts between
classical vs. quantum methods) - Practical applications to quantum domains, such as implementation of AI, or information retrieval (IR) techniques The proceedings include 21 long papers and 3 position papers. Each paper was
thoroughlyreviewedbyatleasttwomembersoftheinternationalProgramC- mittee. The proceedings highlight the cross-disciplinary nature of quantum - teraction with papers covering topics such as computation, cognition, decision
theory, information retrieval, information systems, social interaction, compu- tional linguistics and ?nance. In addition, we were honored to receive a keynote presentation by Dagmar Bruss (Institute for Theoretical Physics,
University of D¨ usseldorf). We gratefully acknowledgethe supportof EarlResearchand Kirsty Kitto for helping prepare these proceedings.