This dissertation examines two interconnected research questions: What conditions give rise and lead to the electoral success of anti-establishment parties and what are the consequences of this electoral success? Literature concerning anti-establishment parties fails to investigate this phenomenon in its entirety by focusing disproportionately on the electoral success of these parties neglecting the consequences of this electoral success. Although the electoral success of anti-establishment parties and the subsequent consequences have different theoretical underpinnings, the effects that anti-establishment parties have on individual parties and the party system are dependent upon the electoral success of these of parties. Therefore, this dissertation focuses on both the electoral success and the consequences of anti-establishment parties in Western Europe. Concerning electoral success, this dissertation offers a new approach to the literature by arguing that anti-establishment parties, regardless of their placement on the political spectrum, are born out of the dissatisfaction towards traditional parties within the electorate. Using quantitative analyses of eighteen Western European countries covering the time period 1970-2005, this dissertation offers a unified analysis of anti-establishment parties, regardless of their placement on the political spectrum, examining the political, social, and economic conditions that give rise to the anti-establishment party phenomenon. The findings indicate that while the factors leading to the emergence of anti-establishment parties may be the same regardless of the placement of these parties on the political spectrum, the factors leading to their electoral success are dependent upon their ideological orientation. Furthermore, the electoral success of these new parties has consequences for other individual parties and the broader party system. This dissertation argues that the existence of these parties alone is not enough to accomplish this aim; these parties must be seen as threats to existing mainstream parties on either the left or the right or in some cases, both. In order to counter the threat from these anti-establishment parties, traditional parties may change their ideological positions or organizational structures. Utilizing qualitative (face-to-face interviews with party elites) and evidence from party manifestos from 1970-2005 in six countries, these analyses indicate that the electoral success of anti-establishment parties affects individual parties by altering the ideological placement, particularly on issues relevant to anti-establishment party electoral success. To a lesser extent, traditional parties alter their organizational structures (i.e., allocating more power to rank-and-file members, regional, and local branches), in order to counter this new electoral threat. Moreover, the electoral success of anti-establishment parties causes instability within the broader party system. Utilizing quantitative, statistical methods to analyze eighteen western European countries between 1970 and 2005, this dissertation finds that the electoral success of anti-establishment parties increases the amount of electoral volatility and the amount of polarization both within the system and between traditional parties. However, anti-establishment parties do not mobilize the electorate leading to increases voter turnout in these eighteen countries. Finally, anti-establishment parties, by gaining seats in national legislatures, upset the traditional coalitional dynamics. As such, the electoral success of anti-establishment parties leads to shorter coalitional governments within the party systems of Western Europe.