Prisons in the United States are operated under strict authority of both the federal and state governments as incarceration is a concurrent power under the Constitution of the United States. Imprisonment is one of the main forms of punishment for the commission of felony offenses in the United States. Less serious offenders, including those convicted of misdemeanor offenses, may be sentenced to a short term in a local jail or with alternative forms of sanctions such as community corrections (halfway house), probation, and/or restitution. In the United States, prisons are operated at various levels of security, ranging from minimum-security prisons that mainly house non-violent offenders to Supermax facilities that house well-known criminals and terrorists such as Terry Nichols, Theodore Kaczynski, Eric Rudolph, Zacarias Moussaoui, and Richard Reid. The United States has the highest documented incarceration rate, and total documented prison population in the world. As of year-end 2006, a record 7.2 million people were behind bars, on probation or on parole. Of the total, 2.2 million were incarcerated. More than 1 in 100 American adults were incarcerated at the start of 2008. The People's Republic of China ranks second with 1.5 million, despite having over four times the population of the US.
Many of the smaller county and city jails do not classify prisoners (that is, there is no separation by offense type and other factors). While some of these small facilities operate as "close security" facilities, to prevent prisoner-on-prisoner violence and increase overall security, others may put many prisoners into the same cells without regard to the criminal histories of the prisoners. Other local jails are large and have many different security levels. For example, one of the largest jails in the United States is in Cook County (located in Chicago). This facility has eleven different divisions (including one medical unit and two units for women prisoners), each classified at a different security level, ranging from dormitory style open housing to super-secure lock-down. In California, to prevent violence, prisoners are segregated by race, ethnicity, and sexual orientation while held in county jails and in the California Department of Corrections and Rehabilitation's reception centers, where newly committed prisoners are assessed prior to being transferred to their "mainline" (long-term) institutions.
Some prisoners are given life sentences. In some states, a life sentence means life, without the possibility of parole. In other states, people with life sentences are eligible for parole. In some cases the death penalty may be applied.
Many legislatures continued to reduce discretion in both the sentencing process and the determination of when the conditions of a sentence have been satisfied. Determinate sentencing, use of mandatory minimums, and guidelines-based sentencing continue to remove the human element from sentencing, such as the prerogative of the judge to consider the mitigating or extenuating circumstances of a crime to determine the appropriate length of the incarceration. As the consequence of "three strikes laws," the increase in the duration of incarceration in the last decade was most pronounced in the case of life prison sentences, which increased by 83% between 1992 and 2003
Supermax prison facilities provide the highest level of prison security. These units hold those considered the most dangerous inmates. These include international and domestic spies, terrorist, inmates who have committed assaults, murders or other serious violations in less secure facilities, and inmates known to be or accused of being prison gang members. The United States Federal Bureau of Prisons operates multiple Supermax facilities, and one such facility that is exclusively Supermax (ADX Florence).
In a maximum security prison or area, all prisoners have individual cells with sliding doors controlled from a secure remote control station. Prisoners are allowed out of their cells one out of twenty four hours. When out of their cells, prisoners remain in the cell block or an exterior cage. Movement out of the cell block or "pod" is tightly restricted using restraints and escorts by correctional officers.
Under close security, prisoners usually have one or two person cells operated from a remote control station. Each cell has its own toilet and sink. Inmates may leave their cells for work assignments or correctional programs and otherwise may be allowed in a common area in the cellblock or an exercise yard. The fences are generally double fences with watchtowers, housing armed guards, plus often a third, lethal-current electric fence in the middle.
Prisoners that fall into the medium security group may sleep in dormitories on bunk beds with lockers to store their possessions. They may have communal showers, toilets and sinks. Dormitories are locked at night with one or more correctional officers supervising. There is less supervision over the internal movements of prisoners. The perimeter is generally double fenced and regularly patrolled.
Prisoners in minimum security facilities are considered to pose little physical risk to the public and are mainly non-violent "white collar criminals". Minimum security prisoners live in less-secure dormitories, which are regularly patrolled by correctional officers. As in medium security facilities, they have communal showers, toilets, and sinks. A minimum-security facility generally has a single fence that is watched, but not patrolled, by armed guards. At facilities in very remote and rural areas, there may be no fence at all. Prisoners may often work on community projects, such as roadside litter cleanup with the state department of transportation or wilderness conservation. Many minimum security facilities are small camps located in or near military bases, larger prisons (outside the security perimeter) or other government institutions to provide a convenient supply of convict labor to the institution. Many states allow persons in minimum-security facilities access to the internet.
The United States has the highest incarceration rate in the world at 737 persons imprisoned per 100,000 (as of 2005). A report released Feb. 28, 2008 indicates that in the United States more than 1 in 100 adults is now confined in an American jail or prison. The United States has 5% of the world's population and 25% of the world's incarcerated population. 
In 2006 the incarceration rate in England and Wales is 139 persons imprisoned per 100,000 residents, while in Norway it is 59 inmates per 100,000, whilst the Australian imprisonment rate is 163 prisoners per 100,000 residents, and the rate of imprisonment in New Zealand last year was 179 per 100,000.
In 2001 the incarceration rate in People's Republic of China was 111 per 100,000 in 2001 (sentenced prisoners only), although this figure is highly disputed. Chinese human rights activist Harry Wu, who spent 19 years in forced-labor camps for criticizing the government, estimates that 16 to 20 million of his countrymen are incarcerated, including common criminals, political prisoners, and people in involuntary job placements. Ten million prisoners would mean a rate of 793 per 100,000.
In August 2003, a Harper's article by Wil S. Hylton estimated that "somewhere between 20 and 40 % of American prisoners are, at this very moment, infected with hepatitis C". Prisons may outsource medical care to private companies such as Correctional Medical Services, which, according to Hylton's research, try to minimize the amount of care given to prisoners in order to maximize profits.
Also identified as an issue within the prison system is gang violence, because many gang members retain their gang identity and affiliations when imprisoned. Segregation of identified gang members from the general population of inmates, with different gangs being housed in separate units often results in the imprisonment of these gang members with their friends and criminal cohorts. Some feel this has the effect of turning prisons into "institutions of higher criminal learning.
Many prisons in the United States are overcrowded. For example, in California, 33 prisons have a capacity of 100,000, but they hold 170,000 inmates. Many prisons in California and around the country are forced to turn old gymnasiums and classrooms into huge bunkhouses for inmates. They do this by placing hundreds of bunk beds next to one another, in these gyms, without any type of barriers to keep inmates separated.
Some advantages of private prisons have been cited. These include flexibility, including the ability to terminate a contract more easily and cost-effectively than it would be to close down a government prison and lay off civil servants in the event of a decline in prison population. Private prisons also have an incentive to look for ways to save on costs; for instance, Travis Snelling of the Corrections Corporation of America notes that his prisons are designed to save on labor, which represents 70% of the total costs over the useful life of a prison. This is particularly important given that posts must often be manned 24 hours a day, requiring more than 5 employees to cover all the shifts. Snelling estimates: "If you can eliminate one post by your architectural design, just one, that'll save you well over $100,000 in a given marketplace, as far as labor is concerned.
The three leading corporations in the private prison business in the U.S. are the Corrections Corporation of America, the GEO Group, and Cornell Companies.
Private companies which provide services to prisons combine in the American Correctional Association, which advocates legislation favorable to the industry.
Under U.S. law convicted felons lose their eligibility to apply for Medicare and Medicaid. Housing one prisoner costs a state between $18,000 and $31,000 annually, $33 per day for the average prisoner and $100 per day for an elderly prisoner. Most DOCs report spending more than 10 percent of the annual budget on elderly care. State governments pay all of their inmates' housing costs which significantly increase as prisoners age.
Some have criticized the United States for incarcerating a large number of non-violent and victimless offenders; half of all persons incarcerated under state jurisdiction are for non-violent offences, and 20% (in State prisons, whereas Federal prison percentages are higher) are incarcerated for drug offences. "Human Rights Watch believes the extraordinary rate of incarceration in the United States wreaks havoc on individuals, families and communities, and saps the strength of the nation as a whole." The population of inmates housed in prisons and jails in the United States exceeds 2 million, with the per capita incarceration population higher than that officially reported by any other country. Because of its size and influence, the U.S. prison industry is often referred to as the prison-industrial complex. Criminal justice policy in the United States has also been criticized for the disproportionate representation of African Americans and other minorities.
Reporting at the annual meeting of the American Sociological Association (August 3, 2008), Becky Pettit, associate professor of sociology from the University of Washington and Bryan Sykes, a UW post-doctoral researcher, reveal that the mammoth increase in the United States’ prison population since the 1970s is having profound demographic consequences that disproportionately affect black males. Drawing data from a variety of sources that looked at prison and general populations, the researchers found that the boom in prison population is hiding lowered rates of fertility and increased rates of involuntary migration to rural areas and morbidity that is marked by a greater exposure to and risk of infectious diseases such as tuberculosis and HIV or AIDS.