Parens patriae describes the inherent power of the state, usually exercised by the courts, to act as guardian of those who cannot protect their own interests, particularly children and adults lacking capacity. It underpins the court’s role in making welfare decisions where an individual is vulnerable or unable to decide for themselves. In England and Wales, and Northern Ireland, the concept informs the family courts’ and High Court’s inherent jurisdiction over children, and complements statutory regimes such as the Children Act 1989 and Mental Capacity Act 2005. In Ireland, it underlies the High Court’s inherent jurisdiction in child protection and certain medical treatment and wardship cases, alongside legislation such as the Child Care Act 1991 and Assisted Decision-Making (Capacity) Act 2015. In Scots law, similar protective functions arise through the parens patriae jurisdiction of the Court of Session and statutory frameworks for children and adults with incapacity, rather than through a single codified definition. Across the UK and Ireland, parens patriae is primarily a judicial, common law doctrine, shaping decisions on custody, medical treatment, wardship, deprivation of liberty and other protective interventions in the best interests of the person concerned.