Political decentralization in Europe and elsewhere during the last several decades triggered a substantial body of research on the causes and the effects of this important development. In the arena of party politics, a critical question is how political parties react to the challenges and opportunities presented by multi-tiered or multilevel states, defined here as states where political power resides at various territorial levels, or within these states how national (statewide) and regionalist (nationalist or autonomist) parties respond to each other. Researchers have begun to investigate how national or statewide parties organize, campaign, make policy and govern at different state levels. Today we have a better understanding of the complex strategic challenges faced by statewide parties in the more complex environment. We know less about regionalist parties. Therefore, this panel places regionalist parties at the center of the analysis, and addresses the relationship between decentralization and regionalist party electoral strength (Massetti & Schakel), decentralization as a national-level strategy to confront the regionalist party challenge (Meguid), the factors that drive regionalist parties to go beyond their niche to pursue catch-all electoral strategies (Elias & Tronconi), a methodology for coding regionalist parties’ center-periphery preferences as reflected in their manifestos (Gómez, Alonso & Cabeza), and the factors that influence regionalist parties’ incentives to support a national-level government (Field). Consequently, it will contribute greatly to building knowledge of territorial politics in Europe.