Green parties exist in almost all Western European party systems. They are represented in several national, regional, and local parliaments. This study discusses the reasons for the Green parties' electoral success. Six sets of variables which claim to explain the political success of the Greens in the 1980s are introduced and measured: the socio-economic system, the political system, the party system, the extraparliamentary power, the values and attitudes of citizens, and the intraparty organization of the Greens.