For decades now, West European politics has witnessed radical changes. Mainstream parties, once oligarchs in remarkably stable political systems, are experiencing a profound challenge to their legitimacy so that the question of their decline, or even their death, has often been raised hand in hand with the question of the crisis of representative democracy. Political commentators and scholars generally assume that party changes such as those made by many mainstream parties are simply to be seen within the context of structural changes in political demand. However, the literature overlooks an essential part of the story by taking the profound structural changes in the environment in which political parties operate as the main argument to explain their evolution over time. This dissertation argues that it is possible to better understand the when, why and how mainstream parties evolved over time through the use of the concept of (bundle of) party reform(s) and by bringing together lessons drawn from the literatures on party change, institutional reform and political marketing.This dissertation proposes a research strategy based on a mix of innovative methods. Three empirical steps were followed in order to provide ground for the new theoretical approach I propose to party reform as well as to help explain party engineering in mainstream parties. The structure of this dissertation builds in particular on the opposition between the rational-institutionalist approach to party change and its constructivist variant. I decided to focus this study on the six mainstream Belgian parties because of the distinctive context Belgium offers for comparatively examining the resilience of political parties that are particularly rooted at the core of a political system but which consistently face strong challenges. The first empirical part of the dissertation consist in building a descriptive inventory of all the reforms implemented by the Belgian mainstream parties on their political product between 1987 and 2014. It engages with ...