A reformatory or reformatory school is a youth detention center or an adult correctional facility popular during the late 19th and early 20th centuries in Western countries. In the United Kingdom and United States, they came out of social concerns about cities, poverty, immigration, and gender following industrialization, as well as from a shift in penology to reforming instead of punishing the criminal. They were traditionally single-sex institutions that relied on education, vocational training, and removal from the city. Although their use declined throughout the 20th century, their impact can be seen in practices like the United States' continued implementation of parole and the indeterminate sentence.

Reformatory schools were penal facilities originating in the 19th century that provided for juvenile offenders. They were certified by the government starting in 1850. As society's values changed, the use of reformatories declined and they were coalesced by an act of Parliament^[which?] into a single structure known as approved schools. Although similar in some of their practice, industrial schools were intended to prevent vulnerable children becoming criminal.

There was a perceived rise in juvenile delinquency in the early 19th century; whereas in a rural economy very young children could gain paid employment doing tasks such as bird scaring and stone gathering. These opportunities were not available in the cities. Youngsters were very visible on the streets. In 1816, Parliament set up a ‘Committee for Investigating the Alarming Increase in Juvenile Crime in the Metropolis’, in 1837 the writer Charles Dickens published Oliver Twist, a story about a child involved in a street gang, and in 1847 it was recognised in the Juvenile Offences Act 1847 that juvenile offenders under 14 should be tried in a special court and not an adult court. Begging and vagrancy was rife, and it was these low level misdemeanours that caused the magistrates to send vulnerable youngsters to industrial schools to learn to be industrious, and learn skills that would make then more employable.

More serious crimes required an element of punishment, but in an environment away from older prisoners who would have a negative effect on the youngster. The power to set up such an establishment was given in the Youthful Offenders Act 1854 (also known as the Reformatory Schools Act 1854). This provided financial assistance and support for reformatory schools for convicted young offenders as an alternative to prison. Industrial schools were regularised three years later by the Industrial Schools Act 1857.

Early initiatives began in 1756, with the founding of the Marine Society "for the purpose of clothing landsmen and boys for the use of the king's ships, and as an expedient to provide for poor boys who might become a nuisance." In 1788, the Philanthropic Society was set up and opened an institution at St George's Fields, Southwark, "for the protection of poor children, and the offspring of convicted felons; and for the reformation of those who have themselves been engaged in criminal practices."

Another institution was also begun in 1778, on the Isle of Wight, which originally served as a military hospital and a children's asylum. By 1838, it had been transformed into HM Prison Parkhurst, a prison for children. 123 Parkhurst apprentices were sent to the Colony of New Zealand in 1842 and 1843, and nearly 1,500 boys between the ages of 12 and 18 were sent to various colonies in Australia and New Zealand. The Swan River Colony received 234 such apprentices between 1842 and 1849, then chose to accept adult convicts as well. Victoria and Tasmania also received "Parkhurst Boys", who were referred to as "apprentices", not convicts. During this period, Parkhurst Prison Governor Captain George Hall (in office from 1843 to 1861) employed the boys to make bricks for the construction of the C and M block wings of the prison building.

In 1817, Samuel Hoare the Quaker banker and philanthropist and the architect James Bevan proposed a new suitable prison for young offenders; but there was no consensus as to whether a prison should be for deterrence, retribution, punishment, removing a cause of nuisance, or reforming the prisoner.

Separation of youngsters from adult prisoners was proposed again by Robert Peel's Gaols Act 1823, but implementation generally failed. From 1824 to 1826 some boys were housed on the prison hulk Captivity.