This volume contains thirty-one papers presented at the Twelfth Scientific Meeting of the IFIP Working Group on Reliability and Optimization of Structural Systems which took place in Aalborg, Denmark, from May 22-25, 2005. The Working Group Conference was organized by the IFIP (International Federation for Information Processing) Working Group 7.5 of the Technical Committee on Modelling and Optimization. The purpose of the Working Group is to promote modern structural system reliability and optimization theory and its applications, to stimulate research, development and application of structural system reliability and optimization theory, to assist and advance research and development in these fields, to further the dissemination and exchange of information on reliability and optimization of structural systems, and to encourage education in structural system reliability and optimization theory.