This book offers a critical overview on the literature on party change and provides original data on several dimensions of party organizations, focusing in particular on Portuguese political parties. The Portuguese case study will be used to illustrate how political parties evolve and the main differences in the trajectory experienced by parties in old democracies. Therefore, starting from the main theoretical contributions used to study party change, this book examines some key dimensions of the role played by political parties: ideological and programmatic orientations, the social basis of support, party organization, electoral campaigns and the elections of party leaders. Through a wide and rich data collection and the comparative perspective adopted, this book furthers our understanding of how Portuguese political parties have changed and the impact of this change on the quality of democracy.