||Since the 1970s, there has been an increase in the number of niche parties that are concerned with a specific issue or cluster of highly related issues such as minority containment/cultural protection and the environment in the countries of Western Europe. A similar trend is also now apparent in many Central and Eastern European states. While the creation of niche parties is not, by itself, necessarily of great importance to a party system, what is important is when these parties exhibit success at the ballot box. The electoral fortunes of niche parties can have important effects on party systems by upsetting mainstream party relationships and long-standing governing coalitions, and influencing public policy. Thus, the central question of this project is under what conditions do niche parties achieve electoral success? This dissertation provides a better understanding of how institutions, strategy, and socioeconomic conditions explain variations in the electoral fortunes of niche parties. More specifically, this project expands upon the strategic explanation, by incorporating agency for niche parties, and tests the strategic interaction model, one that looks at how the strategies of mainstream and niche parties play out in a given institutional and socioeconomic context. Employing a mixed-methods research design of statistical analysis and in-depth interviews from Denmark, France, Hungary, and the Netherlands, the results reveal, for example, that the electoral fortunes of niche parties are often not enhanced through avenues such as participating in an electoral alliance or governing coalition, and that voters and mainstream parties seemingly respond to niche parties and issues based on different factors.