Political parties regularly change and adapt in response to ever-changing circumstances. Until now these changes have frequently prompted both scholars and the media to suggest a whole new type of political party, and over time the number of models and types has proliferated to the point of confusion, contradiction, and a loss of explanatory power. In this sophisticated yet accessible study, André Krouwel rejects this mélange of models as inadequate. He utilizes a wide range of data sources to analyze the ideological, organizational, and electoral change undergone by more than one hundred European parties in fifteen different countries, from Scandinavia to the Iberian Peninsula, between 1945 and 2010. The result is one of the most comprehensive empirically grounded studies to date of the genesis, development, and transformation of political parties in advanced democratic states.