In recent years, increasing attention has been devoted to electoral turmoil associated with the emergence of new political actors or the crisis of established parties at national level. However, in multi-level systems, transformations have also affected sub-national politics. This article seeks to understand some hitherto unexplored aspects of political change in regional party systems. Change is linked to the concept of instability and operationalized in terms of electoral volatility. To account for the fact that instability may be driven by distinct regional and national pressures coexisting in regional elections, a new measure of volatility has been developed. It is disaggregated into two territorial components: ‘Region-specific volatility’ (RSV) and ‘Region-transcending volatility’ (RTV). RSV refers to changes in electoral support for political parties competing exclusively in one region, while RTV is calculated for parties that ‘transcend’ regional boundaries – i.e. they are electorally active in several/all regions of a country or are part of institutionalized inter-regional networks. By applying this new measure to 385 elections in 58 European regions, this article shows that levels and types of instability have varied significantly over time, across regions and across countries. This may also account for different developments in territorial politics observed in Western Europe.