The United States and Soviet Union signed the Strategic Arms Reduction Treaty (START) on July 31, 1991. After the demise of the Soviet Union in December 1991, the parties signed a Protocol that named the four former Soviet Republics with nuclear weapons on their territory—Ukraine, Belarus, Kazakhstan, and Russia—parties to the Treaty. START entered into force on December 4, 1994. The Treaty was to remain in force for 15 years, unless replaced by a subsequent agreement, and, therefore, expired on December 5, 2009. According to Article XVII of the Treaty, the parties must meet "no later than one year" before this date to consider whether the Treaty should be extended or allowed to lapse. If the parties had agreed to extend the Treaty, the extension would last five years, unless START were replaced by a subsequent agreement during that time. The United States and Russia held several meetings in the latter years of the Bush Administration to discuss the options for continuing their bilateral arms control relationship after START, but did not reach an agreement on whether to extend START or on how to replace it. The Obama Administration resumed the discussions in May 2009 and concluded the New START Treaty in March 2010. The United States and Russia signed the new Treaty on April 8, 2010. The Obama Administration has also pledged to negotiate another Treaty, after New START, which will impose deeper reductions on U.S. and Russian nuclear weapons. The United States and Soviet Union negotiated the original START Treaty between 1984 and 1991. It contains many detailed definitions and restrictions that not only limit the permitted number of nuclear warheads but also restrain the locations and movement of delivery vehicles carrying nuclear warheads and require extensive exchanges of data about them. Many of these provisions reflect the competitive relationship between the United States and Soviet Union evident at the time, and the concerns that drove their inclusion in the Treaty may no longer seem as important to the U.S.-Russian relationship. For example, some officials in the Bush Administration and analysts in the broader foreign policy community argued that, because the United States no longer structures its nuclear forces in response to a Russian threat, it no longer needed a treaty that restrains and reduces the weapons that make up that threat. They, therefore, questioned whether the START Treaty, or U.S.-Russian nuclear arms control in general, remained important as tools in the political relationship between the United States and Russia. Some U.S. critics of arms control have, therefore, argued that the bilateral arms control process should fade away after START expired. They note that START may have served its purpose by helping to reduce the size of the Russian arsenal after the demise of the Soviet Union and by restraining the permitted operations of the remaining forces, but its reductions have been overtaken by deeper cuts mandated by the 2002 Moscow Treaty. Moreover, they note that a treaty that restricts Russian forces will also serve to restrict the flexibility of U.S. forces. In the current environment, they contend, the United States may be better served by maintaining its own freedom of action in deploying and operating its nuclear forces than by retaining START or negotiating similar restraints on U.S. and Russian forces. Others, however, argue that START and the arms control process remain relevant to the U.S.-Russian relationship and that START should be replaced with a similar treaty. In this view, the predictability and transparency created by START's well-defined restrictions on Russian and U.S. nuclear forces can enhance the security of both parties. Moreover, continuing the cooperation fostered by the arms control process can help to restore some trust in the relationship between the two nations. In addition, some in Russia still feel threatened by U.S. nuclear weapons and continue to value the restraints provided by arms control treaties. Some Members of Congress joined this debate, with several endorsing the view that extending START, and its monitoring and verification provisions, would help improve the relationship between the United States and Russia. For example, Senator Richard Lugar stated that "the current U.S.-Russian relationship is complicated enough without introducing more elements of uncertainty. Failure to preserve the START Treaty would increase the potential for distrust between the two sides." Some also believe, as Senator Lugar has noted, that the "failure to renew START will be seen worldwide as weakening the international nuclear nonproliferation regime and a further sign to many foreign leaders and experts that U.S. nonproliferation policy is adrift." Others in Congress, however, feel that a rush to complete a new START Treaty could undermine U.S. security by leading to restrictions and compromises that interfere with the U.S. ability to maintain a credible nuclear deterrent. Congress has limited influence on the process of seeking a replacement for START. If the United States and Russia negotiate a new treaty, the Senate will have to provide its advice and consent before the parties ratify the Treaty. However, if the two parties do not reach any agreement and START lapses, the Senate would not have to approve or reject the outcome. Nevertheless, Congress can, through resolutions, hearings, and consultations, offer the Administration its views on the future of the START Treaty and the U.S.-Russian arms control process. This report provides background information about the START Treaty and reviews the discussions and probable provisions in the successor, New START Treaty. START limits long-range nuclear-capable delivery systems—land-based intercontinental ballistic missiles (ICBMs), submarine-launched ballistic missiles (SLBMs), and heavy bombers—in the United States and the four states of the former Soviet Union. The Treaty limits both the number of delivery systems and the number of warheads carried on these systems. As Table 1 below indicates, each side can deploy 6,000 "attributed" warheads on no more than 1,600 ballistic missiles and heavy bombers, with no more than 4,900 attributed warheads on land-based and submarine-based ballistic missiles. Within the aggregate limits on ballistic missile warheads, START also limits each side to no more than 1,540 warheads on heavy ICBMs, which are defined as those with a throwweight greater than 4,350 kilograms, and 1,100 warheads on mobile ICBMs. These two limits are an added effort to restrain forces that the United States feared would provide the Soviet Union with an avenue to exceed the warhead limit. The United States had long sought to use the arms control process to limit, or eliminate, the Soviet monopoly on heavy ICBMs because it believed that the Soviet Union could expand the capabilities of these missiles by deploying them with more or higher yield. The United States did not have any ballistic missiles of this size, and had no plans to develop or deploy them. The Soviet Union initially resisted U.S. pressures to limit these missiles, but eventually agreed to halve their force of 304 SS-18 ICBMs, each of which was deployed with 10 warheads, under START. As the START negotiations proceeded through the 1980s, the United States also grew concerned about the Soviet deployment of ballistic missiles on mobile launchers. The Soviet Union had begun to deploy single-warhead SS-25 missiles on road-mobile launchers and 10-warhead SS-24 missiles on rail-mobile launchers. The United States considered these missiles both a military and an arms control problem. Because the United States did not think it could locate and track these missiles all the time, it believed it would be difficult to target them during a conflict. Moreover, because the Soviet Union had large land areas where it could operate and conceal these missiles, U.S. negotiators argued that the United States would not be able to monitor mobile ICBM deployments well enough to count the missiles and verify Soviet compliance with the limits in START. The United States initially proposed that START ban mobile ICBMs, even though it was considering the possible use of mobile launchers for its new 10-warhead Peacekeeper (MX) ICBM and for a prospective small, single-warhead ICBM. But, after the United States and Soviet Union began to consider options for a monitoring and verification regime that might track the numbers of mobile ICBMs, they agreed to limit, rather than ban, these systems. The limited numbers, when combined with location restrictions, notifications prior to movement, data exchanges that identified the numbers of missiles and warheads based at approved locations, and a continuous monitoring regime outside the final assembly facility for one type of mobile ICBM, would help each side count the number of acknowledged mobile ICBMs and complicate efforts to conceal extra missiles or warheads. Even though the United States eventually dropped its plans to deploy mobile ICBMs, it agreed to apply these limits and restrictions to the Peacekeeper (MX) missiles that were deployed in silos. START also limits the total amount of throwweight on each side's ballistic missiles, to an amount equal to around 54% of the amount of throwweight on Soviet missiles before the Treaty entered into force. Throwweight is the combined weight of the post-boost vehicle, warheads, guidance system, penetration aids, and other equipment found on the front end of a missile. It is considered to be a measure of a missile's destructive capacity because larger missiles with greater throwweight can carry larger or greater numbers of warheads. Hence, this limit was a further effort by the United States to limit the potential for the Soviet Union to add warheads to its missiles in violation of the Treaty's limits. Because Soviet forces deployed when START was signed carried had than three times as much throwweight as U.S. missiles, the United States did not have to reduce its forces to comply with this limit. However, the United States could have exceeded the limit on throwweight if it had deployed new, larger missiles while START remained in force. START counts each deployed ICBM and its associated launcher, each deployed SLBM and its associated launcher, and each deployed heavy bomber as a single delivery vehicle under the Treaty limit of 1,600 delivery vehicles.