In the United States Army, United States Air Force, and United States Marine Corps, general is a four-star general officer rank, with the pay grade of O-10. General ranks above lieutenant general and below General of the Army or General of the Air Force; the Marine Corps does not have an established grade above general. General is equivalent to the rank of admiral in the other uniformed services. Since the grades of General of the Army and General of the Air Force are reserved for war-time use only, and since the Marine Corps has no five-star equivalent, the grade of general is currently considered to be the highest appointment an officer can achieve in these three services.
The United States Code explicitly limits the total number of generals that may be on active duty at any given time. The total number of active duty general officers is capped at 302 for the Army, 279 for the Air Force and 80 for the Marine Corps. For the Army and Air Force, no more than 16.3% of the service's active duty general officers may have more than two stars, and no more than 25% of those may have four stars. This corresponds to 12 Army generals, 11 Air Force generals and 3 Marine generals.
Some of these slots are reserved by statute. For the Army and Air Force, the Chief of Staff and the Vice Chief of Staff are both generals; for the Marine Corps, the Commandant and the Assistant Commandant are both generals; for the National Guard, the Chief of the National Guard Bureau  is a general under active duty in the army or air force.
There are several exceptions to these limits allowing more than allotted within the statute. A four-star officer serving as Chairman or Vice Chairman of the Joint Chiefs of Staff does not count against his service's general or flag officer cap. An officer serving as Chief of the National Guard Bureau does not count against his service's general officer cap. An officer serving in one of several joint positions does not count against his service's four-star limit, but he does count against his service's limit on officers with more than two stars; these positions include the commander of a unified combatant command, the commander of United States Forces Korea, and the deputy commander of United States European Command but only if the commander of that command is also the Supreme Allied Commander, Europe. Officers serving in certain intelligence positions are not counted against either limit, including the Director of the Central Intelligence Agency. The President may also add four-star slots to one service if they are offset by removing an equivalent number from other services. Finally, all statutory limits may be waived at the President's discretion during time of war or national emergency.
Four-star grades go hand-in-hand with the positions of office they are linked to, so these ranks are temporary. Officers may only achieve four-star grade if they are appointed to positions of office that require the officer to hold such a rank. Their rank expires with the expiration of their term of office, which is usually set by statute. Generals are nominated for appointment by the President from any eligible officers holding the rank of brigadier general or above, who also meet the requirements for the position, under the advice and/or suggestion of their respective department secretary, service secretary, and if applicable the Joint Chiefs of Staff. For some specific positions, statute allows the President to waive those requirements for a nominee whom he deems would serve national interests. The nominee must be confirmed via majority vote by the Senate before the appointee can take office and thus assume the rank. The standard tour length for most four-star positions is three years, bundled as a two-year term plus a one-year extension, with the following exceptions:
Note: Extensions of the standard tour length can be approved, within statutory limits, by their respective service secretaries, the Secretary of Defense, the President, and/or Congress but these are rare, as they block other officers from being promoted. Some statutory limits under the U.S. Code can be waived in times of national emergency or war. Four-star ranks may also be given by act of Congress but this is extremely rare.
Other than voluntary retirement, statute sets a number of mandates for retirement. Four-star officers must retire after 40 years of service unless he or she is reappointed to serve longer. Otherwise all general officers must retire the month after their 64th birthday. However, the Secretary of Defense can defer a four-star officer's retirement until the officer's 66th birthday and the President can defer it until the officer's 68th birthday.
General officers typically retire well in advance of the statutory age and service limits, so as not to impede the career paths of junior officers. Since only a finite number of four-star slots are available to each service, typically one officer must leave office before another can be promoted. Maintaining a four-star rank is a game of musical chairs; once an officer vacates a position bearing that rank, he or she has no more than 60 days to be appointed or reappointed to a position of equal importance before he or she must involuntarily retire. Historically, officers leaving four-star positions were allowed to revert to their permanent two-star ranks to mark time in lesser jobs until statutory retirement, but now such officers are expected to retire immediately to avoid obstructing the promotion flow.
To retire at four-star grade, an officer must accumulate at least three years of satisfactory active duty service in that grade, as certified by the Secretary of Defense and confirmed by the Senate. The Secretary of Defense may reduce this requirement to two years, but only if the officer is not being investigated for misconduct. Officers who do not meet the service-in-grade requirement revert to the next highest grade in which they served satisfactorily for at least six months. It is extraordinarily rare for a four-star officer not to be nominated to retire in grade or for such a nomination not to be confirmed by the Senate unanimously.
Four-star officers typically step down from their posts up to 60 days in advance of their official retirement dates. Officers retire on the first day of the month, so once a retirement month has been selected, the relief and retirement ceremonies are scheduled by counting backwards from that date by the number of days of accumulated leave remaining to the retiring officer. During this period, termed transition leave or terminal leave, the officer is considered to be awaiting retirement but still on active duty.
|United States commissioned officer and officer candidate ranks|
|(no universal insignia)||(no authorized insignia)|
|Air Force:||CDT / OT||2d Lt||1st Lt||Capt||Maj||Lt Col||Col||Brig Gen||Maj Gen||Lt Gen||Gen||GAF
|Army:||CDT / OC||2LT||1LT||CPT||MAJ||LTC||COL||BG||MG||LTG||GEN||GA
||General of the
|Marine Corps:||Midn / Cand||2ndLt||1stLt||Capt||Maj||LtCol||Col||BGen||MajGen||LtGen||Gen|||||
|Navy:||MIDN / OC||ENS||LTJG||LT||LCDR||CDR||CAPT||RDML||RADM||VADM||ADM||FADM
||Admiral of the
|Public Health Service:||||ENS||LTJG||LT||LCDR||CDR||CAPT||RADM||RADM||VADM||ADM|||||
National Oceanic and
Grade is authorized for use by U.S. Code but has not been created  Grade requires Congressional approval for re-activation 
|United States warrant officer ranks|
|Approximate insignia:||(no universal insignia)||(no universal insignia)||(no universal insignia)||(no universal insignia)||(no universal insignia)|
|Public Health Service:|||||||||||
|National Oceanic and Atmospheric Administration:|||||||||||
Grade is authorized for use by U.S. Code but has not been created