This volume presents the refereed proceedings of the 9th International Conference on Deontic Logic in Computer Science, DEON 2008, held in Luxembourg in July 2008. The 16 revised full papers presented together with 4 invited talks were carefully reviewed and selected for inclusion in the book. The topics addressed are development of formal systems of deontic logic and related areas of logic, and applications. Of particular interest is the interaction between computer systems and their users; the papers focus also on the special topic of logical approaches to deontic notions in computer science in the area of security and trust, encompassing applications in e-commerce as well as traditional areas of computer security.