The aim of this thesis is to investigate the relationship between the claims advanced by regional nationalist parties for the re-organisation of state structures based on the recognition of their distinct national groups and the process of territorial restructuring in Great Britain, Spain and Belgium. The objectives are to examine the conditions under which regional nationalist parties have influenced the reform of state structures and to assess the factors that condition the relation between their electoral and policy success. The thrust of the argument advanced is that the influence of regional nationalist parties on the reform of state structures can be understood as the result of processes that take place during the agenda-setting and the decision-making phases. To provoke institutional change, regional nationalist parties must firstly set the political agenda by exerting pressures on mainstream parties in the electoral, parliamentary and governmental arenas. Secondly, institutional change is likely to occur, if nationality claims are accommodated by mainstream parties that are ideologically open to political decentralisation and the recognition of regional nationalism and if the question of territorial autonomy has a broader resonance for political competition between mainstream parties. Finally, institutional change is likely to occur, if the government undertaking reforms is cohesive and there is no ideological opposition by any partisan veto players to those reforms. These arguments are examined through a comparative-historical analysis of political decentralisation in Great Britain, Spain and Belgium, over a time period spanning the late 1960s to the late 1990s. Each case study is disaggregated into time periods that represent moments in the process of territorial restructuring, bounded by significant changes in the assertiveness of regional nationalist parties and the occurrence of territorial reform. Within each period, the process tying the claims of regional nationalist parties to institutional change is examined. Theses different observations are used as evidence for undertaking a comparison of the accommodation of nationalism across space and time. The thesis finds that regional nationalist parties exert a strong influence on the creation of decentralised state structures when they can set the political agenda. It is also necessary for nationality claims to be accommodated by mainstream parties that are open to the accommodation of nationalism and for them to feature as a salient issue of party competition, and for constitutional reforms to be undertaken by a cohesive centre-left government. The ideological opposition of a partisan veto player in government to institutional change is sufficient for bringing about the failure of reform. The second finding is that the configuration of factors producing reforms tends to diversify with the development of restructuring. During the empowerment of regional governments, regional nationalist parties can set the political agenda and their claims may feature as an issue of party competition, but the reforms must be undertaken by centre-left governments. The subsequent deepening of territorial autonomy is stimulated by regional nationalist demands but undertaken by governments of different ideological persuasions whose mainstream parties may compete on the territorial dimension.