The history of jails in English-speaking countries, including the United States, can be traced back to 12th-century England during the reign of King Henry II, who ordered the construction of jails and placed them under the control of the crown’s local government representative, the county sheriff. Their primary purpose was to detain people awaiting trial and those convicted but awaiting punishment. The earliest reference to U.S. jails is to the construction of a “people pen” in 1632 in pre-Revolutionary Boston. Mirroring the brutal British penal codes and practices of the day, the dominant form of criminal punishment in Colonial America was corporal—with serious crimes punishable by death, physical mutilation, branding, or whipping, and lesser offenses by public ridicule and humiliation through the use of the stocks, the pillory, the public cage, or the ducking stool. But with the conversion of Philadelphia’s Walnut Street Jail into the country’s first penitentiary in 1790—as part of penal reform championed by the Quakers—incarceration as punishment soon became the default response for serious lawbreaking and with it the modern prison system was born.

Today jails are, with few exceptions, municipal or county-level confinement facilities that are administered by local law enforcement agencies or departments of correction. Like their historical antecedents, they are used to detain people awaiting trial who are deemed a flight risk or a danger to public safety. But many also house a range of other people caught up in the criminal system as described below. Jails range in size from small “lockups” that hold no more than a handful of people to networks of facilities, such as the eight jails in Los Angeles County that house approximately 20,000 inmates. Their costs are mainly paid for by a municipality or county, with reimbursements sometimes coming from the state or federal governments.

Unlike state prisons, which almost exclusively hold people serving state sentences, jail populations are heterogeneous, making them particularly challenging to manage.

Jails may house the following people:

• Pretrial detainees, held from the time they are arrested until they post bail, are released on their own recognizance or to some form of pretrial community supervision—or until the cases against them are settled by trial or plea.
• Locally sentenced inmates convicted of minor crimes, serve short custodial sentences, typically a year or less, but longer in some states.
• State-sentenced inmates convicted of more serious crimes may be in jail awaiting transfer to a state prison or are serving their sentence in a local facility due to prison overcrowding. Local jurisdictions are paid to house these overflow inmates. That trend is most significant in California, where the state department of correction is under court order to reduce crowding in prisons.
• Apprehended probation violators may either be awaiting a hearing on an alleged violation of the terms of their supervision in the community or serving the remainder of their sentence in local confinement.
• Apprehended parole violators are often in jail awaiting a hearing on an alleged violation or a transfer back to state prison.
• Pretrial federal detainees may be in jail awaiting trial on federal charges in jurisdictions where no federal detention beds are available. Local jurisdictions are paid to house these inmates.
• Apprehended pretrial or sentenced inmates from other jurisdictions may be awaiting transfer or be housed at the jail due to unavailability of beds in another state or local jurisdiction.
• Immigration and Customs Enforcement detainees are sometimes held at the request of the U.S. government pending deportation or adjudication of immigration violations. Local jurisdictions are paid to house these inmates.