Single Integrated Operational Plan (or SIOP) is a blueprint that tells how American nuclear weapons would be used in the event of war.
The SIOP is created from a conceptual guide issued by the President. The guide is converted by the Secretary of Defense into the Nuclear Weapons Employment Policy (NUWEP) of basic targeting objectives, target lists and operational constraints. The NUWEP is then delivered to the Joint Chiefs of Staff (JCS) and emerges as the Joint Strategic Capabilities Plan (JSCP). The JSCP is then converted into the actual targeting orders, timing and weapon allocation, the SIOP, by STRATCOM. The entire process takes up to 18 months. Under President Clinton the SIOP held four major attack options, 65 limited attack options, and a number of generalised adaptive options for threats originating outside Russia or China.
Nuclear strike targets are listed as the National Target Base (NTB), built from an Intelligence list of 150,000-plus sites across the world. The number of targets in the NTB has varied enormously. It peaked at around 16,000 in 1985, fell to around 12,500 following the collapse of the Soviet Union, dropped to about 2,500 in 1995, before rising to the current list of 3,000 targets. Around 75% of the current targets are in Russia, 1,100 are nuclear weapons sites.
The US nuclear arsenal holds around 7,000 individual warheads. A 'strong' counterforce strike using up to 1,500 warheads would kill around 120 million Russians; a 'limited' countervalue strike of just 200 warheads would kill around 50 million Russians  (http://www.nrdc.org/nuclear/warplan/index.asp).
While the United Kingdom's nuclear deterrent, four Trident Vanguard class submarines, are strictly under UK national control they do have two distinct roles. The first is part of a UK-only retaliatory response to a nuclear attack, whether a full strategic strike involving all of the Royal Navy's Trident submarines, or a limited tactical strike involving perhaps only one missile. The second role is one in which the Royal Navy participates in the SIOP, in effect becoming non-distinct from the U.S. Navy's Trident submarines. This role was to be part of a NATO response to a Soviet nuclear strike. The Royal Navy's contribution to the SIOP shows the power of the nuclear arsenal committed to the plan, the four Vanguard submarines (at maximum capacity) could strike 640 separate targets, and this is a fraction of the U.S. deterrent force.
The Single Integrated Operational Plan is a highly classified document, and has been one of the most secret and sensitive issues in U.S. national security policy.
Currently SIOP plans are named after the fiscal year in which they come into effect, this was first officially applied to SIOP-93, prior to that plans used a two-character alphanumeric designation. A new SIOP is approved every year, although the plan may well be unchanged.
Plans were developed from the immediate post-war period. By the 1950s around 5,500 targets were listed to receive SAC bomber strikes - mainly industrial sites but also 'counterforce' targets. Pressure from the Eisenhower administration, and development of effective ICBM and SLBM systems, forced a more formalised procedure - the SIOP.
The first SIOP was developed in 1960, consisting of a list of targets (the National Strategic Target List, NSTL) and identifying the assets to be used against each target. This first SIOP was extensively revised by a team at the RAND Corporation to become SIOP-62, a massive strike with the entire US arsenal of 3,200 warheads against the USSR, China and Soviet-aligned states, casualty estimates exceeded 250 million. In 1963 the Kennedy administration ordered Robert McNamara to revise this plan, resulting in SIOP-63 - a strong counterforce strategy with a number of options, and the 'no first use' policy became implicit.
Counterforce dominated SIOP plans until SIOP-5 in 1976 when the plan became a model for deterrence, based on Nixon's NSDM-242 and sometimes called the 'Schlesinger Doctrine' after then-Secretary of Defense, James Schlesinger. The ever expanding target lists were split into classes of targets, with a wider range of plans matching strikes to political intentions from counterforce to countervalue, or any mix/withhold strategy to control escalation. The SIOP policy was further modified during the presidency of Carter under Presidential Directive-59, although the 'ethos' remained the same. Under Reagan, through NSDD-13, there was a return to a strong counterforce strategy. Although first-strike was still explicitly removed, the vision was of an extended exchange.
The Creation of SIOP-62: More Evidence on the Origins of Overkill (http://www.gwu.edu/~nsarchiv/NSAEBB/NSAEBB130/)
A Do-It-Yourself SIOP (http://www.thebulletin.org/issues/2001/ja01/ja01lortie.html)