A Unified Combatant Command (UCC) is a United States joint military command composed of forces from two or more services, has a broad and continuing mission, and is organized either on a geographical basis (known as "Area Of Responsibility", AOR) or on a functional basis. All UCCs are commanded by either a four star general or admiral and are considered "joint" commands with specific badges denoting their affiliation. UCCs (formerly known as "COCOMs", a term now reserved exclusively for the authority they hold, which is also called "combatant command") are led by Combatant Commanders (CCDRs), formerly known as a regional "Commander-in Chief" (CINC; pronounced "Sink").
The Unified Command Plan (UCP) is updated annually in conjunction with the DoD Fiscal Year and can modify areas of responsibility or combatant command alignments or assignments. As of January 2008, there were ten Unified Combatant Commands as specified in Title 10 and the latest annual UCP. Six have regional responsibilities, and four have functional responsibilities.
The Goldwater-Nichols Defense Reorganization Act of 1986 clarified and codified responsibilities that CINCs and their predecessors (theater or area commanders) had undertaken since World War II, and which were first given legal status in 1947.
The U.S. Atlantic Command became the Joint Forces Command in the 1990s after the Soviet threat to the North Atlantic had disappeared and the need rose for an integrating and experimentation command for forces in the continental United States.
Regional CINCs were created in order to have a local supreme commander who could exercise unified command and control across service boundaries, ideally eliminating or diminishing interservice rivalries. CINCs reported directly to the United States Secretary of Defense, and through him to the President of the United States. One of the best known CINCs was Norman Schwarzkopf, commander of U.S. Central Command (CENTCOM) during Operation Desert Storm.
On 24 October 2002, Secretary of Defense Donald H. Rumsfeld announced that in accordance with Title 10 of the US Code (USC), the title of "Commander-in-Chief" would thereafter be reserved for the President, consistent with the terms of Article II of the United States Constitution. Thereafter, the military CINCs would be known as "combatant commanders", as heads of the Unified Combatant Commands.
Discussion of adding a sixth geographical command with an AOR aligned to Africa (which had previously been assigned to three separate Combatant Commands) began before the departure of Secretary of Defense Donald Rumsfeld. This sixth geographical combatant command for Africa (USAFRICOM) was approved 6 February 2007.
U.S. Africa Command was established in October 2007 and operated under U.S. European Command during its first year. October 1, 2008 marked U.S. Africa Command's transition to independent Unified Command Status. It is now focused on synchronizing hundreds of activities inherited from three regional commands that previously coordinated U.S. military relations in Africa.
Each combatant command is headed by a four star general or admiral selected by the Secretary of Defense and President and confirmed by Congress. Goldwater-Nichols also resulted in specific Joint Professional Military Education (JPME)http://www.jfsc.ndu.edu/schools_programs/rcjpme/overview.asp#jpme requirements for officers before they could attain flag or general officer rank thereby preparing them for duty in Joint assignments such as UCC staff or Joint Chiefs of Staff assignments, which are strictly controlled tour length rotations of duty. However, in the decades following enactment of Goldwater-Nichols, these JPME requirements have yet to come to overall fruition. This is particularly true in the case of senior naval officers, where sea duty/shore duty rotations and the culture of the naval service has often discounted PME and JPME as a measure of professional development for success. Although slowly changing, the JPME requirement still continues to be frequently waivered in the case of senior admirals nominated for these positions 
The chain of command runs from the President to the Secretary of Defense to the combatant commanders of the Unified Combatant Commands. The Chairman of the Joint Chiefs of Staff may transmit communications to the Commanders of the Unified Combatant Commands from the President and Secretary of Defense and advises both on potential courses of action, but does not exercise direct military command over any combatant forces. Under Goldwater-Nichols, the service chiefs (also four stars in rank) are charged with the responsibility to "organize, train and equip" forces for use by the combatant commands and do not exercise any operational control over their forces.
Each combatant command can be led by a general or flag officer from any of the services. Most commands have traditional service affiliations, but in recent years, non-traditional appointments have become more common. EUCOM was traditionally an Army command with USAF generals on occasion, but was held by a Marine from 2003 through 2006. CENTCOM was traditionally an Army and Marine command but the last commander was a Navy admiral. PACOM has always been commanded by a Navy admiral due to the wide expanse of ocean, although Air Force generals have been nominated for the post. U.S. Atlantic Command (USACOM) was also a traditional Navy assignment until it was successively commanded by Marine, Army, and Air Force generals, thereby becoming the first to have had commanders from all four services (USACOM was redesignated as JFCOM in 1999). CENTCOM and SOUTHCOM were traditionally Army general positions until the Marines received their first CinC assignments. This led the way for General Pace, a Marine, to become the first Marine Vice Chairman of the Joint Chiefs of Staff and ultimately Chairman. CCDRs are strong candidates for either position.
At some points, there have been proposals to create some sort of Reserve Affairs Worldwide Support command for the Reserve and National Guard of the United States. Also considered was the United States Medical Command which was proposed but then abandoned.