Thisvolumecontainsthe30regularpapers, the11shortpapersandtheabstracts oftwoinvited keynotesthatwerepresentedatthe12thInternationalConference on Principles of Distributed Systems (OPODIS) held during December 15-18, 2008 in Luxor, Egypt. OPODIS is a yearly selective international forum for researchers and prac- tioners in design and development of distributed systems. This year, we received 102 submissions from 28 countries. Each submission was carefully reviewed by three to six Program Committee members with the help of external reviewers, with 30 regular papers and 11 short papers being selected. The overall quality of submissions was excellent and there were many papers that had to be rejected because of organization constraints yet deserved to be published. The two invited keynotes dealt with hot topics in distributed systems: "The Next 700 BFT Protocols" by Rachid Guerraoui and "On Rep- cation of Software Transactional Memories" by Luis Rodriguez. On behalf of the Program Committee, we would like to thank all authors of submitted papers for their support. We also thank the members of the Ste- ing Committee for their invaluable advice. We wish to express our apprec- tion to the Program Committee members and additional external reviewers for their tremendous e?ort and excellent reviews. We gratefully acknowledge the Organizing Committee members for their generous contribution to the s- cess of the symposium. Special thanks go to Thibault Bernard for man- ing the conference publicity and technical organization. The paper submission and selection process was greatly eased by the EasyChair conference system (http: //www. easychair. org).