His Majesty's Young Offenders Institution (Her Majesty's Youth Offenders Institution if the reigning monarch is female) (or HM YOI) are youth detention centres for offenders between ages 15 to 21 in the United Kingdom. These offenders will have received a custodial sentence following criminal offence convictions or may be being held on remand awaiting trial on pending charges.
A young offender's institution can accommodate between 60 and 400 young people, with each age group housed in its own wing of between 30 and 60 inmates. Most young offenders' institutions are run by the national prison service while some are run by private companies on behalf of the prison service.