Welcome to the proceedings of the 4th International Symposium on Parallel and Distributed Processing and Applications (ISPA 2006), which was held in Sorrento, Italy, December, 4-6 2006. Parallel computing has become a mainstream research area in computer s- ence and the ISPA conference has become one of the premier forums for the p- sentation of new and exciting research on all aspects of parallel and distributed computing. We are pleased to present the proceedings for ISPA 2006, which comprises a collection of excellent technical papers and keynote speeches. The accepted papers cover a wide range of exciting topics including architectures, languages, algorithms, software, networking and applications. The conference continues to grow and this year a record total of 277 ma- scriptsweresubmitted forconsiderationbytheProgramCommittee.Fromthese submissions the Program Committee selected only 79 regular papers in the p- gram, which re?ects the acceptance rate as 28%. An additional 10 workshops complemented the outstanding paper sessions. The submission and review process worked as follows. Each submission was assigned to at least three Program Committee members for review. Each P- gram Committee member prepared a single review for each assigned paper or assigned a paper to an outside reviewer for review. In addition, the Program Chairs and Program Vice-Chairs read the papers when a con?icting review - sult occurred. Finally, after much discussion among the Program Chairs and ProgramVice-Chairs, based on the review scores, the ProgramChairs made the ?nal decision. Given the large number of submissions, each ProgramCommittee member was assigned roughly 7-12 papers.