The study aims to explain differences in the levels of regionalism both between different regions and within individual regions over time. The existing literature focuses on globalisation and European integration as the main causes of changes in regionalism across time, treating the phenomenon mainly as a part of broader political developments. By overlooking internal developments in individual regions in this way, it is impossible to explain differences between regions. Another main strand of the literature focuses on cultural and ethnic differences, but these differences tend to be relatively static and thus unsuitable for explaining variation within individual regions over time. Instead, the thesis looks for answers to these questions in regional economies. One major hypothesis is that economically strong but politically peripheral regions will be better equipped to challenge the central state and have stronger incentives to desire control over their own resources. A quantitative study across 212 Western European regions seeks to test the relationship between regionalism and a set of common explanatory variables, including the economic strength of the region. Variables such as cultural distinctiveness, geographical position, economic development, globalisation and Europeanisation are tested for their effects on the regional identities expressed by the population. The findings of the quantitative study form the basis of a model of the causes of regionalism, which is further explored through case studies of Scotland and Rogaland, two regions that have experienced growth across time both in terms of economic development and globalisation. On this basis, the model predicts a growth in regionalism across time in both regions. These predictions are tested and confirmed, and an exploratory qualitative study examines why economic development and globalisation may have led to growth in regionalism in the two regions