A Green party or ecologist party is a formally organized political party based on the principles of Green politics. These principles usually include social justice, reliance on grassroots democracy, nonviolence, and an emphasis on environmentalism. "Greens" believe that the exercise of these principles leads to world health. The platform of a green party can run from the centre to the far-left, however most run to the centre-left, embracing social-democratic economic policies and forming coalitions with leftists.

There are distinctions between "green" parties and "Green" parties. Any party, faction, or politician may be labeled "green" if it emphasizes environmental causes. Indeed, the term may even be used as a verb: it is not uncommon to hear of "greening" a party or a candidate.

In contrast, formally organized Green parties may follow a coherent ideology that includes not only environmentalism, but often also other concerns such as social justice, consensus decision-making, and pacifism. Greens believe that these issues are inherently related to one another as a foundation for world peace. The best-known statement of the above Green values is the Four Pillars of the Green Party, adopted by the German Greens in 1979-1980 (but forsaken since). The Global Greens Charter lists six guiding principles which are ecological wisdom, social justice, participatory democracy, nonviolence, sustainability and respect for diversity.

Jill Stein (born 1950) is an American physician and candidate for President of the United States in 2012 with the Green Party of the United States. Stein was a candidate for Governor of Massachusetts in the 2002 and the 2010 gubernatorial elections. Stein is a resident of Lexington, Massachusetts and a 1979 graduate of Harvard Medical School. She serves on the boards of Greater Boston Physicians for Social Responsibility and MassVoters for Fair Elections, and has been active with the Massachusetts Coalition for Healthy Communities.

In October 2011, Stein announced her candidacy for the presidential nomination of the Green Party in the 2012 general election.

Stein advocates for the creation of a "Green New Deal", the objective of which would be to employ "every American willing and able to work" to address "climate change...[and the] converging water, soil, fisheries, forest, and fossil fuel crises" by working towards "sustainable energy, transportation and production infrastructure: clean renewable energy generation, energy efficiency, intra-city mass transit and inter-city railroads, “complete streets” that safely encourage bike and pedestrian traffic, regional food systems based on sustainable organic agriculture, and clean manufacturing of the goods needed to support this sustainable economy". The initial cost of the Green New Deal would be funded by various mechanisms, including "taxing Wall Street speculation, off shore tax havens, millionaires and multimillion dollar estates" as well as a 30% reduction in the U.S. military budget. She cites a study of the economic effects of the 1930s New Deal projects by Dr. Phillip Harvey, Professor of Law & Economics at Rutgers School of Law as academic evidence for the Green New Deal.