if a party system is really a system, i.e. a set of objects with relationships between the objects and between their attributes, and not simply an unordered set of political parties, then it is a vital problem in party research to identify thesystemsproperties of this kind of system. We argue that the semantically relevant properties of the party systems in European democracies may be derived from the observational outcome of the operation of a party system, i.e. the national elections since the introduction of democratic procedure. Thus we arrive at five systems dimensions: functional orientation, fractionalization, radical orientation, polarization and volatility. The problems of party system change and stability are analysed by estimating the occurrence of trends and fluctuations over time in these party system dimensions for the set of European democratic party systems. The finding is that the widely accepted thesis of stability in European party systems is not corroborated.