Detain describes the act of lawfully depriving a person of their liberty for a period of time, usually by the police, immigration authorities or other authorised bodies, without necessarily bringing immediate criminal charges. It typically involves preventing a person from leaving a place, such as a police station, prison, immigration removal centre, hospital or port of entry.Across England and Wales, Scotland, Northern Ireland and Ireland, detention powers are governed by statute (for example, Police and Criminal Evidence Act 1984, Criminal Justice (Scotland) Act 2016, Garda Síochána powers, immigration and mental health legislation), supplemented by case law on false imprisonment, unlawful detention and human rights (Article 5 ECHR).Key legal issues include: the source and scope of the power to detain; maximum detention periods; procedural safeguards (such as access to legal advice, notification of rights, judicial oversight and review); and remedies for unlawful detention. In criminal practice, “detain” often refers to custody for questioning or charge; in public law it commonly concerns immigration detention, mental health detention, and judicial review of deprivation of liberty. Usage and underlying concepts are broadly consistent across the UK and Ireland, though precise statutory frameworks and terminology differ.