Comparative political science has largely ignored the marked cross-national variation in Green party electoral performance. This article uses a unique aggregate dataset of 347 parliamentary elections from 32 countries over the course of 45 years to test competing theories about the causes of Green party success. The findings show that voter demand, institutions and mainstream party strategy all affect the Green vote. Green parties do well in societies with post-materialist conflicts caused by high levels of wealth or the presence of a tangible environmental dispute. The article also shows that regional decentralisation helps Green parties, but electoral systems have little effect on their vote share. Most importantly, it demonstrates that the impact of mainstream party strategy on Green electoral strength is dependent on the age of the Green party. While mainstream parties can undermine young Green parties by adopting the environmental issue, this effect is reversed once the Greens have survived a number of elections. Thus 'accommodative' mainstream party strategies eventually boost the Green vote by increasing the salience of the key Green issue. KEYWORDS Green parties; niche parties; environmentalism; party competition; post-materialism The Greens are the most enduring and cohesive new party family since the rise of the Social Democrats a century ago. From Italy to Ireland, Finland to France, Green parties have been regular participants in governing coalitions across developed democracies. And whether in government or not, many Green parties have blackmail or coalition kingmaker potential (Sartori 1976: 121-4). However, such success has not been universal. In some advanced democraciesfor example, Norway, Spain and Poland -Green parties remain fundamentally inconsequential electoral players.