DOD spends billions of dollars annually to sustain its weapon systems to support current and future operations. The Air Force and Navy are operating many of their fixed-wing aircraft well beyond their original designed service lives and therefore are confronted with sustainment challenges. Chapter 1 examines the trends in availability and O&S costs for selected Air Force and Navy fixed-wing aircraft since fiscal year 2011, including whether they met availability goals, and assesses the extent that the departments documented sustainment strategies, reviewed sustainment metrics, and implemented plans to improve aircraft availability. Software is integral to the operation and functionality of DOD equipment, platforms, and weapon systems, including tactical and combat vehicles, aircraft, ships, submarines, and strategic missiles. Chapter 2 examines the extent to which (1) DOD has policies and organizations in place to manage the sustainment of operational system software for weapon systems; and (2) DOD and the military departments track costs to sustain weapon system software. The Department of Defense (DOD) and the Department of Energy (DOE) are undertaking an extensive, multifaceted effort to sustain and modernize U.S. nuclear weapons capabilities, including the nuclear weapons stockpile; the research and production infrastructure; delivery systems; and the nuclear command, control, and communications (NC3) system. Chapter 3 presents observations on the extent to which the FY 2018 joint report provides accurate and complete information about nuclear sustainment and modernization budget estimates and related budget estimating methodologies. The Department of Defense and NNSA have sought for nearly a decade to replace the capabilities of the aging W78 nuclear warhead used by the U.S. Air Force. Chapter 4 describes NNSA's steps in key early planning areas ... including program management, technology assessment, and coordination with facilities and capabilities ... to prepare to restart a program to replace the W78. Responsibility for U.S. nuclear weapons resides in both the Department of Defense (DOD) and the Department of Energy (DOE). DOD develops, deploys, and operates the missiles and aircraft that deliver nuclear warheads. It also generates the military requirements for the warheads carried on those platforms. Chapter 5 focuses on the facilities managed by the DOE and its semiautonomous National Nuclear Security Administration (NNSA). The Trump Administration's Nuclear Posture Review includes plans for the United States to deploy two new types of nuclear weapons "to enhance the flexibility and responsiveness of U.S. nuclear forces." Chapter 6 highlights that these weapons represent a response to Russia's deployment of a much larger stockpile of lower-yield nonstrategic nuclear weapons.