This volume, which is also available online from http://booksonline.iospress.nl/, contains the papers that have been presented at ECAI 2012, the Twentieth European Conference on Artificial Intelligence. ECAI was held in the beautiful city of Montpellier (France) from August 27 to August 31, 2012. It is the biennial Conference on Artificial Intelligence (ECAI) organised by the European Coordinating Committee for Artificial Intelligence (ECCAI) and the premier forum for presenting AI results in Europe. ECAI is the place for researchers and practitioners of Artificial Intelligence to gather and to discuss the latest trends and challenges in all subfields of AI as well as to demonstrate innovative applications and uses of advanced AI technology.

As in past editions, ECAI 2012 also featured the STarting AI Researcher Symposium (STAIRS) and the Conference on Prestigious Applications of Intelligent Systems (PAIS) as sub-conferences and a special System Demonstrations Track. The proceedings of PAIS 2012 and the System Demonstrations Track are included in this volume, while those of STAIRS 2012 have been published as a separate volume with IOS Press.

At ECAI 2012, we celebrated several anniversaries: the Turing centennial, the twentieth ECAI conference and twenty-five years of AI Communications (the journal on Artificial Intelligence that has a close relationship with ECCAI). The celebrations took the form of a special track where a number of distinguished speakers provided a historical perspective on the field of artificial intelligence in Europe and beyond with accompanying papers to appear in a special issue of AI Communications on this topic. In addition, ECAI 2012 featured four keynote speakers, an extensive workshop program, seven invited tutorials and the novel Frontiers of Artificial Intelligence track. In this track, six invited speakers delivered perspective talks on particularly interesting new research results, directions and trends in Artificial Intelligence or in one of its neighboring fields.

A total of 563 papers (among which 71 short papers) was submitted to the main technical track of ECAI 2012, a number that is similar to that of the past three editions. Submissions in all areas of Artificial Intelligence were received though the areas of Knowledge Representation & Reasoning, Machine Learning and Multi-Agent Systems attracted the largest number of submissions as usual. After review, 140 long and 23 short papers were (sometimes conditionally) accepted for presentation and included in the proceedings. Rejected long papers were not considered for the short paper category. One paper was withdrawn, which results in a final acceptance rate of 28,5% for long papers and 32,3% for short ones. The acceptance rate for long papers is slightly higher than usual due to the option of conditionally accepting papers.

Special thanks go to the workshop chairs, Jérôme Lang and Michèle Sebag, for attracting an extensive and exciting workshop program, to Gerhard Brewka, Michael Wooldridge, Maria Fox and the ECCAI board for advice on the program, to Albrecht Zimmermann for assistance with confmaster and of course to all the invited speakers (keynote, tutorial, frontiers of AI track, and Anniversary and Turing session), to the PAIS, STAIRS and System Demonstrations Track chairs, to the area chairs and PC members, the reviewers, the local organizing committee, the sponsors and all authors who have submitted their work to ECAI.

Luc De Raedt, Christian Bessiere and Didier Dubois, June 2012