This thesis undertakes an empirical exploration of multi-level party politics in Italy and Spain since the 1990s, with a particular focus on the regional level of party organisation and electoral competition. It finds that statewide parties have adopted different territorial strategies to confront the common challenge of multi-level coordination in a decentralised political system. Regional branches of statewide parties increasingly respond to the competitive pressures emerging from sub-national party systems, rather than the preferences of the national leadership, although the interests of national and regional elites may often coincide. Regional party systems in both countries are diverging in their structures of competition, not only from the national level but also from each other. This is closely related to the different types of electoral challenge posed by the main non-statewide parties in these regions. The methodological design consists of two national frameworks and four regional case studies, two in Italy (Campania, Lombardia) and two in Spain (Andalusia, Galicia). These are linked through the use of comparable empirical indicators over a similar timeframe. The national frameworks required the compilation of a new data-set on regional elections in Italy and Spain, a detailed analysis of party statutes and their evolution, and a full exploration of the secondary literature in different languages. The regional case studies required extensive archival analysis of the main national newspapers and their regional editions, reinforced by a series of in-depth interviews with political actors in all four regions. The case studies found strong empirical evidence concerning the distinctive character of presidentialism at regional level; the continued importance of clientelism in shaping political relations at subnational levels; the pursuit of autonomist strategies by regional branches of statewide parties; the variety of competitive strategies adopted by non-statewide parties; the role of regional arenas as battlegrounds for national factional disputes; and the importance of local coalition testing for subsequent coalitional choices at regional level. The comparative conclusions serve to consolidate these findings, as well as to reflect on further avenues for research in this rapidly developing field.