Underground infrastructure is essential for delivery of services that support a strong urban economy and high quality of life. However, current underground engineering practice and education do not promote the interdisciplinary and systems-oriented development approaches that support urban sustainability. Further, lack of long-term investment and coordination of underground development and engineering at most levels of U.S. government curtails cost effectiveness and sustainability and jeopardizes future U.S. technological leadership in underground engineering. Ad hoc development of underground infrastructure projects adversely affects sustainability and increases infrastructure costs. Comprehensive lifecycle planning of underground space as part of a three-dimensional, integrated above- and belowground urban system could reduce costs and increase sustainability. Better analysis and design approaches that facilitate long-term asset management also support sustainability. Retrospective cost analyses of existing infrastructure can inform future triple-bottom-line cost assessments and quantify the economic, social, and environmental benefits of different infrastructure choices. Targeted long-term research, better interdisciplinary education and training for underground engineers, and greater societal awareness of the importance of the underground are needed to maximize the contributions of underground engineering to sustainability. This paper summarizes conclusions from the National Research Council report Underground Engineering for Sustainable Urban Development (2013; http://www.nap.edu/catalog.php?record-id=14670).