This paper seeks to enhance our understanding of the ways in which decentralizing reforms influence and shape party politics at both the statewide and non-statewide level, looking at the special case of asymmetric devolution. In asymmetric devolution, statewide political parties have to adapt to the presence of new arenas of political competition at the subcentral level, but these new arenas are either not present, or not equally developed, in the whole state territory. The paper hypothesizes possible party reactions and adaptations to these circumstances, focusing on changes in party organization, interparty cooperation and interparty competition. It then goes on to assess the emerging reality of party politics under asymmetric devolution in two prominent Western European cases: Spain, where asymmetric devolution has developed over the past two decades, and the United Kingdom, where devolution has been established more recently. This comparative analysis illustrates the organizational strains that asymmetric devolution creates for political parties, and assesses its impact on party competition and government formation at the statewide level. [ABSTRACT FROM AUTHOR]