The Standing Committee on Defense Materials Manufacturing and Infrastructure (DMMI) conducted a workshop on July 23-24, 2012, to share information and gather perspectives on issues concerning Materials and Manufacturing Capabilities for Sustaining Defense Systems. This workshop, held at the headquarters building of the National Academies, 2101 Constitution Avenue N.W., Washington D.C., was conducted according to the procedures of the National Research Council (NRC) for a convening activity. That is, all workshop participants-including presenters, members of the DMMI standing committee, Reliance 21, invited guests, and visitors-spoke as individuals, and no overall findings, conclusions, or recommendations were developed during or as a result of the workshop. All statements and views summarized in this publication are attributable only to those individuals who expressed them. It is worth noting that the sponsor, Reliance 21, is a Department of Defense group of professionals that was established to enable the DOD science and technology (S&T) community to work together to enhance Defense S&T programs, eliminate unwarranted duplication, and strengthen cooperation among the military services and other DOD agencies. The DMMI standing committee named a workshop planning group to develop the workshop agenda and decide on invited guests and presenters, in accordance with the statement of task approved by the Governing Board of the NRC. The planning group also consulted with the Reliance 21 materials and processing community of interest. The presentations and discussions during the workshop are summarized sequentially in the main part of this report. As an aid to readers, nine themes have been identified by the author that recurred in multiple presentations and discussions. Materials and Manufacturing Capabilities for Sustaining Defense Systems: Summary of a Workshop explains these nine themes and summarizes the two day workshop.