Los Alamos National Laboratory (the Laboratory) is the nation's premier national security science laboratory. Its mission is to develop and apply science and technology to ensure the safety, security, and reliability of the United States (U.S.) nuclear stockpile; reduce the threat of weapons of mass destruction, proliferation, and terrorism; and solve national problems in defense, energy, and the environment. The fiscal year (FY) 2013-2037 Twenty-Five Year Site Plan (TYSP) is a vital component for planning to meet the National Nuclear Security Administration (NNSA) commitment to ensure the U.S. has a safe, secure, and reliable nuclear deterrent. The Laboratory also uses the TYSP as an integrated planning tool to guide development of an efficient and responsive infrastructure that effectively supports the Laboratory's missions and workforce. Emphasizing the Laboratory's core capabilities, this TYSP reflects the Laboratory's role as a prominent contributor to NNSA missions through its programs and campaigns. The Laboratory is aligned with Nuclear Security Enterprise (NSE) modernization activities outlined in the NNSA Strategic Plan (May 2011) which include: (1) ensuring laboratory plutonium space effectively supports pit manufacturing and enterprise-wide special nuclear materials consolidation; (2) constructing the Chemistry and Metallurgy Research Replacement Nuclear Facility (CMRR-NF); (3) establishing shared user facilities to more cost effectively manage high-value, experimental, computational and production capabilities; and (4) modernizing enduring facilities while reducing the excess facility footprint. Th is TYSP is viewed by the Laboratory as a vital planning tool to develop an effi cient and responsive infrastructure. Long range facility and infrastructure development planning are critical to assure sustainment and modernization. Out-year re-investment is essential for sustaining existing facilities, and will be re-evaluated on an annual basis. At the same time, major modernization projects will require new line-item funding. This document is, in essence, a roadmap that defines a path forward for the Laboratory to modernize, streamline, consolidate, and sustain its infrastructure to meet its national security mission.