The concept of federalism, and examples of federalism, are considered. Although, over 100 years ago, British Imperial federalists argued the empire must ‘federate or disintegrate’, the imperial federation movement did not succeed. Nor did its close relation, the ‘Home Rule All Round’ movement. Instead, the UK has become a devolved State, which transfers power from the centre without relinquishing sovereignty. The main obstacles to federalism in the UK appear to be the ‘England problem’ and parliamentary sovereignty. It has long been recognized that the disproportionate size of one federal unit can destabilize federalism by affecting the capacity of other territorial units to influence central government. However, the main obstacle lies with an insistence on preserving absolute parliamentary sovereignty in the UK. Dicey insisted that ‘limited sovereignty’ was a ‘contradiction in terms’ and that federalism was ‘absolutely foreign to the historical and, so to speak, instinctive policy of English constitutionalists.’ But this chapter argues there is no reason why sovereignty must be understood only as unlimited and indivisible. The obstacle is a lack of political will and not legal impossibility.