This chapter explains the development of the Westminster party system. It explores how the classification of party systems in the UK has become more complicated since the introduction of elections to the European Parliament and devolved institutions. Party systems are closely related to the electoral system in operation. An alternative way of identifying the number of significant political parties operating in a political system is to measure the effective number of electoral parties (ENEP). The use of proportional representation has given fuller expression to multi-party politics in the devolved assemblies, whereas the simple plurality electoral system used for Westminster elections gave Labour a clear majority of seats in Scotland and Wales at least until the 2015 general election, when the position in Scotland turned radically against Labour and produced something which looked like a one-party system in terms of seats.