This dissertation presents a comprehensive analysis of the factors affecting nationalist parties’ support for the EU over time and across regions. Investigating how European integration and nationalist parties interconnect and looking for patterns among a large number of cases are important to understanding the nature of nationalism and the future of the EU. The approach of looking for patterns among a large number of cases contributes to overcoming limitations of existing studies that are based on a small number of cases and allows more valid generalizations about the nature of nationalist parties in the European context. This study combines a big-N method with case studies. The big-N method relies on aggregated data over a 30-year period from 1984 to 2014 in both Western and Eastern EU member countries to investigate the attitudes and behaviors of all kinds of contemporary nationalist parties toward European integration. A total of 105 nationalist parties across 26 EU member states are included. I build on Arnold, Sapir, and De Vries’s (2012) model for predicting Western European parties’ EU positions based on their electorate, parties, and party system characteristics. The differences are that (1) I focus strictly on nationalist parties, while Arnold, Sapir, and De Vries included all parties, (2) I broaden the scope to include both Western and Eastern European countries, and (3) I have added a variety of country-level variables. In particular, I emphasize party-level variables and party system characteristics, but other country-level variables are also included as controls because these might matter too. A variety of possible variables are classified into two categories: party-level (including ideology, party type (statewide versus minority nationalist), party size, and incumbency) and country-level (including party system fractionalization and polarization, as well as East versus West, state size, immigrant population, the length of EU membership, economic conditions, and public opinion). I look separately at statewide nationalist parties and minority nationalist parties, in addition to all nationalist parties combined, in order to investigate whether the independent variables have different effects for different types of nationalist parties. I found that extreme nationalist parties are more negative toward the EU, minority nationalist parties are more positive than statewide nationalist parties, larger nationalist parties are more positive, incumbent nationalist parties are more positive, political context matters differently for statewide nationalist parties and minority nationalist parties, and characteristics of the state matter more for minority nationalist parties than for statewide nationalist parties. Along with the big-N method, I conducted three case studies in order to investigate the actual changes in strategies and behaviors within real-life political contexts and to access similarities and differences between strategies of different parties in a comparative context.