This volume contains the papers presented at the 11th International Conference on Logic for Programming, Arti?cial Intelligence, and Reasoning (LPAR), held from March 14 to 18, 2005, in Montevideo, Uruguay, together with the 5th - ternational Workshop on the Implementation of Logics (organized by Stephan Schulz and Boris Konev) and the Workshop on Analytic Proof Systems (or- nized by Matthias Baaz). The call for papers attracted 77 paper submissions, each of which was - viewed by at least three expert reviewers. The ?nal decisions on the papers were taken during an electronic Program Committee meeting held on the Internet. The Internet-based submission, reviewing, and discussion software EasyChair, provided by the second PC co-chair, supported each stage of the reviewing p- cess. But the most important work was, of course, done by the 34 PC members and their external reviewers, who provided high-quality reviews. After intense discussions to resolve con?icts among the reviewers, the Program Committee decided to accept 33 papers. The conference program also included 4 invited talks, by Jurgen ..Giesl, Al- ander Leitsch, Helmut Seidl, and Igor Walukiewicz, which are documented by short or extended abstracts in these proceedings. In addition, Mart' ?n Abadi held a tutorial on Reasoning About Security Protocols, and Ian Horrocks on Description Logic Reasoning.