Devolution describes a set of constitutional arrangements where some of the power to make laws and decisions has been transferred by a central parliament or government to institutions in one or more territories within the country. In the UK, bespoke systems of devolution were introduced for Scotland, Wales and Northern Ireland, each based on a law passed by the UK parliament. England is the only territory of the UK without its own law-making powers; the UK parliament continues to make all laws for England.

When it was introduced, UK devolution was described as ‘a process not an event’. Each set of arrangements has seen change since 1999, especially to transfer more powers. Although the extent of devolved powers varies between Scotland, Wales and Northern Ireland, they mostly cover what are sometimes called domestic areas of policy, including most public services. All three operate under what is called a 'reserved powers' model. This means that the devolved institutions are free to make decisions in any area, except those that are listed as specifically reserved to the UK parliament. Reserved powers include foreign affairs, defence, immigration, pensions, and most of the powers over the economy and taxes.