Defence date: 29 April 2016 Examining Board: Professor Hanspeter Kriesi, European University Institute, Supervisor; Professor Stefano Bartolini, European University Institute; Professor Christoffer Green-Pedersen, Aarhus University; Professor Thomas Poguntke, Heinrich Heine University Düsseldorf. Political parties tend to compete over a wide range of issues in their electoral campaigns. Although the choice of issues that parties make can, to a great extent, be explained by several well-established theories (e.g. ownership, median voter), in recent decades the patterns of party issue competition in European democracies have become more complex. Current theories fall short of explaining fully why parties choose to compete over certain issues but not others, and what motivates them to follow issue strategies that either converge or diverge from that of their competitors. Approaching this question from the perspective of polity vs. policy issue competition, this thesis aims to identify the factors that increase the salience of polity issues and shape the patterns of convergence and divergence on polity/policy issue competition. I argue that institutional factors, such as party system competitiveness and left/right polarisation, along with dominant cultural values, the state of the economy, and a party's position within the given party system all influence political parties' behavior in polity/policy issue competition. In order to test this hypothesis, I have conducted quantitative panel-data analysis (with the random effects and fixed effects models for the sample of 18 Western European countries between 1979 and 2013) and produced five case studies (pre-election campaigns in the United Kingdom in 1979, 1997 and 2010, and in Italy in 1979 and 2008). The results of this study suggest that, as predicted, in more competitive and polarised (along the left/right axis) party systems, political parties tend to emphasise polity-type issues, as well as to diverge more sharply on polity/policy issue competition. Equally strong results also appeared for the factor of the party's position within the relevant party system – minor and non-governing parties compete over polity issues more fiercely than their counterparts in power. Finally, while economic factors proved likely to be the key driving force for the salience of polity issues in party competition, the results did not indicate that dominant cultural values produced any effect on polity/policy issue competition.