This book offers a systematic and far-reaching account of party system institutionalization in Western Europe. Drawing upon a wide array of data and through a comparison of 20 countries from the end of WWII to 2019 across three arenas of party competition (electoral, parliamentary, and governmental ones), the empirical analysis shows that, over the past decade, the level of institutionalization in the Western European party systems has dramatically declined compared with previous decades. Electoral, parliamentary, and – in some cases – governmental instability and unpredictability have reached record-high levels. Although the impact of the 2008 Great Recession has certainly worked as a catalyst, this process of de-institutionalization has been mainly driven by long-term factors, such as cleavage decline and length of democratic experience. Moreover, its consequences are relevant not only for the relationship between parties and voters, but also for the very quality of democracy, asparty system deinstitutionalization causes a decline in the citizens’ satisfaction of the way democracy works and even an erosion of the ‘objective’ democratic standards. In a nutshell, Western Europe, once seen as the land of stability and the cradle of democracy, may have become the land of party system deinstitutionalization and incipient democratic backsliding.