During the Cold War, the U.S. nuclear arsenal contained many types of delivery vehicles for nuclear weapons, including short-range missiles and artillery for use on the battlefield, medium-range missiles and aircraft that could strike targets beyond the theater of battle, short- and medium-range systems based on surface ships, long-range missiles based on U.S. territory and submarines, and heavy bombers that could threaten Soviet targets from their bases in the United States. The short- and medium-range systems are considered nonstrategic nuclear weapons and have been referred to as battlefield, tactical, and theater nuclear weapons. The long-range missiles and heavy bombers are known as strategic nuclear delivery vehicles. In 1990, as the Cold War was drawing to a close and the Soviet Union was entering its final year, the United States had more than 12,000 nuclear warheads deployed on 1,875 strategic nuclear delivery vehicles. As of July 1, 2009, according to the counting rules in the original Strategic Arms Reduction Treaty (START), the United States had reduced to 5,916 nuclear warheads on 1,188 strategic nuclear delivery vehicles. Under the terms of the 2002 Strategic Offensive Reduction Treaty (known as the Moscow Treaty) between the United States and Russia, this number was to decline to no more than 2,200 operationally deployed strategic nuclear warheads by the end of 2012. The U.S. Department of State reported that the United States has already reached that level, with only 1,968 operationally deployed strategic warheads in December 2009. The New START Treaty, signed by President Obama and President Medvedev on April 8, 2010, reduces those forces further, to no more than 1,550 warheads on deployed launchers and heavy bombers. According to the U.S. Department of State, on September 1, 2018, the United States had 1,398 warheads on 659 deployed ICBMs, SLBMs, and heavy bombers, within a total of 800 deployed and nondeployed ICBMs, SLBMs, and heavy bombers. Although these numbers do not count the same categories of nuclear weapons, they indicate that the number of deployed warheads on U.S. strategic nuclear forces has declined significantly in the decades following the end of the Cold War. Yet, nuclear weapons continue to play a key role in U.S. national security strategy, and the United States does not, at this time, plan to either eliminate its nuclear weapons or abandon the strategy of nuclear deterrence that has served as a core concept in U.S. national security strategy for more than 60 years. In a speech in Prague on April 5, 2009, President Obama highlighted "America's commitment to seek the peace and security of a world without nuclear weapons." But he recognized that this goal would not be reached quickly, and probably not in his lifetime. And, even though the President pledged to reduce the roles and numbers of U.S. nuclear forces, the 2010 Nuclear Posture Review noted that "the fundamental role of U.S. nuclear weapons, which will continue as long as nuclear weapons exist, is to deter nuclear attack on the United States, our allies, and partners." Moreover, in the 2010 NPR and in the June 2013 Report on the Nuclear Employment Guidance of the United States, the Obama Administration indicated that the United States would pursue programs that would allow it to modernize and adjust its strategic forces so that they remained capable in coming years. The Trump Administration emphasized its continuing support for the U.S. nuclear arsenal in its Nuclear Posture Review, which was released on February 2, 2018. This document notes that "the current threat environment and future uncertainties now necessitate a national commitment to maintain modern and effective nuclear forces, as well as the infrastructure needed to support them." The report not only reaffirms the basic contours of the current U.S. force structure and the ongoing modernization programs, it also calls for the development of a new low-yield warhead for deployment on Trident II (D-5) missiles. Most Members of Congress have supported the general contours of U.S. nuclear posture. While some programs have been open to scrutiny, Congress has continued to support funding for most aspects of the ongoing modernization programs. Nevertheless, questions about the costs of these programs, and the trade-offs they might require within the defense budget, have surfaced in recent years, particularly following the passage of the Budget Control Act in 2011. These concerns may receive additional attention in the 116 th Congress. While Senator James Inhofe, who will likely chair the Senate Armed Services Committee, has offered strong support for the nuclear modernization programs, Representative Adam Smith, who is likely to chair the House Armed Services Committee, has noted that "the current $1.5 trillion plan to build new nuclear weapons and upgrade our nuclear weapons complex is unrealistic and unaffordable." This report reviews the ongoing programs that will affect the expected size and shape of the U.S. strategic nuclear force structure. It begins with an overview of this force structure during the Cold War, and summarizes the reductions and changes that have occurred since 1991. It then offers details about each category of delivery vehicle—land-based intercontinental ballistic missiles (ICBMs), submarine launched ballistic missiles (SLBMs), and heavy bombers—focusing on their current deployments and ongoing and planned modernization programs. The report concludes with a discussion of issues related to decisions about the future size and shape of the U.S. strategic nuclear force. Since the early 1960s the United States has maintained a "triad" of strategic nuclear delivery vehicles. The United States first developed these three types of nuclear delivery vehicles, in large part, because each of the military services wanted to play a role in the U.S. nuclear arsenal. However, during the 1960s and 1970s, analysts developed a more reasoned rationale for the nuclear "triad." They argued that these different basing modes had complementary strengths and weaknesses. They would enhance deterrence and discourage a Soviet first strike because they complicated Soviet attack planning and ensured the survivability of a significant portion of the U.S. force in the event of a Soviet first strike. The different characteristics might also strengthen the credibility of U.S. targeting strategy. For example, ICBMs eventually had the accuracy and prompt responsiveness needed to attack hardened targets such as Soviet command posts and ICBM silos, SLBMs had the survivability needed to complicate Soviet efforts to launch a disarming first strike and to retaliate if such an attack were attempted, and heavy bombers could be dispersed quickly and launched to enhance their survivability, and they could be recalled to their bases if a crisis did not escalate into conflict. According to unclassified estimates, the number of delivery vehicles (ICBMs, SLBMs, and nuclear-capable bombers) in the U.S. force structure grew steadily through the mid-1960s, with the greatest number of delivery vehicles, 2,268, deployed in 1967. The number then held relatively steady through 1990, at between 1,875 and 2,200 ICBMs, SLBMs, and heavy bombers. The number of warheads carried on these delivery vehicles increased sharply through 1975, then, after a brief pause, again rose sharply in the early 1980s, peaking at around 13,600 warheads in 1987. Figure 1 displays the increases in delivery vehicles and warheads between 1960, when the United States first began to deploy ICBMs, and 1990, the year before the United States and Soviet Union signed the first Strategic Arms Reduction Treaty (START). The sharp increase in warheads in the early 1970s reflects the deployment of ICBMs and SLBMs with multiple warheads, known as MIRVs (multiple independent reentry vehicles). In particular, the United States began to deploy the Minuteman III ICBM, with 3 warheads on each missile, in 1970, and the Poseidon SLBM, which could carry 10 warheads on each missile, in 1971. The increase in warheads in the mid-1980s reflects the deployment of the Peacekeeper (MX) ICBM, which carried 10 warheads on each missile. In 1990, before it concluded the START Treaty with the Soviet Union, the United States deployed a total of around 12,304 warheads on its ICBMs, SLBMs, and heavy bombers. The ICBM force consisted of single-warhead Minuteman II missiles, 3-warhead Minuteman III missiles, and 10-warhead Peacekeeper (MX) missiles, for a total force of 2,450 warheads on 1,000 missiles. The submarine force included Poseidon submarines with Poseidon C-3 and Trident I (C-4) missiles, and the Ohio-class Trident submarines with Trident I, and some Trident II (D-5) missiles. The total force consisted of 5,216 warheads on around 600 missiles. The bomber force centered on 94 B-52H bombers and 96 B-1 bombers, along with many of the older B-52G bombers and 2 of the new (at the time) B-2 bombers. This force of 260 bombers could carry over 4,648 weapons. During the 1990s, the United States reduced the numbers and types of weapons in its strategic nuclear arsenal, both as a part of its modernization process and in response to the limits in the 1991 START Treaty. The United States continued to maintain a triad of strategic nuclear forces, however, with warheads deployed on ICBMs, SLBMs, and bombers. According to the Department of Defense, this mix of forces not only offered the United States a range of capabilities and flexibility in nuclear planning and complicated an adversary's attack planning, but also hedged against unexpected problems in any single delivery system.