This volume collects a subset of the high-quality papers that were accepted for presentationatthe20thInternationalSymposiumonComputerandInformation Sciences (ISCIS). ISCIS 2005 was organized by the Department of Computer Engineering of Bo? gazi, ci University and was held in Istanbul, Turkey during 26-28October,2005. Following on the success of previous ISCIS symposia, ISCIS 2005 attracted a total of 491 papers. The submitted papers went through a rigorous subm- sion period with the help of the Program Committee and external reviewers. Keeping up with the tradition of previous editions of ISCIS, the selected papers covera wide spectrum of topicsin computer and information sciences.The main topics of ISCIS 2005 can be listed as: computer networks, sensor and satellite networks,securityand cryptography,performanceevaluation,databasesystems, algorithms,multiagentsystems,e-commerce,machinelearning,naturallanguage processing and information retrieval, image and speech processing, and theory of computing. In addition to the paper presentations, the symposium enjoyed four invited talks. The honorary chair of ISCIS 2005, Erol Gelenbe from Imperial College, London, gave a talk on a probability model of natural or arti?cial infection. The second talk was by Onn Shehory of IBM Haifa Research Labs, on online auctions. JacquesStern from EcoleNormaleSup' erieure,France, focused his talk on alternatives to RSA. The last invited speaker, Yanghee Choi from Seoul National University, Korea, delivered a talk on location and address issues in ubiquitous computing.