The 18th International Conference on Engineering Design, ICED11, was held August 15-18th 2011 at The Technical University of Denmark (DTU), Copenhagen. The Conference is the flagship event of the Design Society, a society dedicated to contributing to a broad and established understanding of development and design. The ICED series of conferences has a long tradition, which started in 1981 with the first ICED in Rome. A total of 419 papers were presented at ICED11, each double-blind reviewed by multiple reviewers. The papers included research papers and case studies on a variety of topics concerned with design thinking, theory, and practice, with a premium placed on evidence-based research. The papers are published in a total of ten volumes of Proceedings, in addition to electronic publication. This volume is concerned with Design Theory and Research Methodology, and contains 38 papers on topics ranging from underlying design theories to design research methodologies and case studies of design research in action.