Who are they?
The Green Party was formed in the 1970s on a platform of ecology, pacifism, and energy reform. They also count equal rights for ethnic minorities and disabled people among their core values. Despite being Europe's most successful environmental party, they are taking a battering in the polls.
Who's their leader?
Jürgen Trittin, 59, holds a degree in Social Sciences from Göttingen, and worked as a publicist, freelance journalist and as a lab assistant, before joining the Greens in 1980.
He entered the regional parliament of Lower Saxony in 1985, and later served as the region's minister for federal and European affairs.
When Schröder's red-green coalition took power in the Bundestag in 1998, Trittin was made minister of environment, nature protection and atomic safety, a post he held until they left office in 2005.
What's their strategy?
The Greens' main election policies correspond to those of the Social Democrats (SPD): the introduction of a minimum wage and measures to improve minimum working conditions, as well as replacing the currently ruling Union's child benefit policy with new state-run nurseries.
They campaign for these policies alongside their core values of prioritizing renewable energy and stamping out animal exploitation.
However, in a step beyond the SPD platform, they advocate reforming the healthcare system to address a perceived double standard between private and state healthcare, as well as radical reform of the tax system with fairness in mind.
This expansion from their traditional manifesto of ecological policies has not broadened support for Trittin's party. Since their peak of 28 percent in April 2011, the Greens recently dropped 17 points to 9 percent, their lowest score in four years.
Manfred Güllner, boss of pollsters the Forsa institute, attributes this slump to their departure from traditional green policies to issues like social inequality. "This issue doesn't fit with them", he said.
The Greens also provoked ridicule in August with plans to introduce a “Veggie Day” in canteens in which meat would be banned.
Why do they matter?
The Greens are polling at between nine and 11 percent and are likely coalition partners for the SPD, given their common policy aims and history of cooperation. This makes them important because without Green support, the SPD will either have to accept defeat or try and form a coalition with Merkel's Union.
The Christian Democratic Party (CDU)