Unions take a variety of forms and it is not always clear what species of legal entity one is dealing with. They need not require a unitary State and may exist along a spectrum of State connectivity, from an affiliation of separate States, through to their complete fusion. This chapter examines other Unions – either past, for example Austria-Hungary, or present, such as the USA and EU. The UK Union itself only dates from 1707, is voluntary in nature, and (unlike some Constitutions) there is no legislation prohibiting secession by one part of the UK, nor any constitutional provision asserting and mandating constitutional integrity. This chapter argues it is preferable to understand the UK as a ‘Union State’ – that is, a State where the centre does not directly control every part of the territory – rather than unitary in nature, because there are different constitutional arrangements in different parts of the UK. Unions tend to do better if more elastic and less constraining. But current understandings of UK parliamentary sovereignty make such elasticity very difficult for the UK Union. The doctrine of unlimited sovereignty places an almost insurmountable barrier to resolution of issues threatening the Union today, particularly those of devolution.