This special issue contains four papers that have been presented at the 13th International Conference on Principles and Practice of Constraint Programming (CP 2007), which was held in Providence, RI, USA, September 23–27, 2007, in conjunction with the International Conference on Automated Planning and Scheduling (ICAPS). Held annually, the CP conference series is the premier international conference on constraint programming. All 165 papers submitted to the conference (143 submissions as research papers and 22 as application papers) received three reviews and were discussed extensively by the members of the Program Committee. From the 52 papers accepted for presentation at the conference, I selected a dozen of papers as candidates for the best paper award, based on the reviews they received. This small set of papers were even further discussed by a subcommittee consisting of Javier Larrosa, Christophe Lecoutre, Christian Schulte and myself. If only one paper finally received the best paper award, it had been chosen from a set of five outstanding papers that were very hard to separate. I thus chose these five papers to be expanded into journal form and submitted to this special issue. One of them declined the invitation because one of the authors was the editor in chief of this journal. The four other papers are those that appear in this issue. The four papers address topics of Constraint Programming that are all very active and promising, such as clause learning, constraint propagation via clause generation, visualisation in constraint-based systems, or solution counting. I would like to thank Javier Larrosa, Christophe Lecoutre and Christian Schulte, the members of the best paper subcommittee, who accepted the intensive task of reading all candidate papers. Thank you to the reviewers, who carefully read and proposed improvements to the four papers published here. Finally, thank you to the authors, who worked hard to address the recommendations of the reviewers for improving even more their excellent papers.