One reason that established political parties are in decline across Europe is the rise of small, single-issue parties devoted to radical right-wing, green, or ethnoterritorial causes. Some of these niche parties succeed; others fail. What explains their diverging fortunes? Meguid's core insight is that their success is due less to their intrinsic appeal than to the strategic choices made by big mainstream parties in their competition with one other. Following the logic of "the enemy of my enemy is my friend," mainstream parties permit small parties to rise so that these draw voters away from their main competitors. This insight allows Meguid to explain both why parties fail in hospitable contexts (think of the unsuccessful Swedish Green Party) and why they succeed in seemingly inauspicious circumstances (the Front National in France). Her conclusions are confirmed by data spanning three decades and by case studies of the French right, the British Green Party, and the Scottish National Party.