Caroline Lucas hopes to become the Greens' first MP
The Green Party effectively began life after a Coventry couple got a group of friends together to discuss a Playboy article on rainforest destruction.
Out of that 1973 meeting a small party called People was born which fielded just five candidates in the February 1974 general election.
The following year People became the Ecology Party and adopted the "Manifesto for a Sustainable Society".
The document would eventually become the "philosophical and policy" blueprint for the Green Party which came into being in 1985.
Funded by its own membership, the Greens long opted not to have a leader, instead being represented by two "principal speakers".
However in 2008 it ditched the policy - which some argued was difficult to explain - and elected Caroline Lucas.
The party has managed to build a respectable representation at local level - they are the second biggest party locally in Norwich, with 13 councillors, and joint second biggest in Lancaster and Brighton.
It got its highest vote share - 14.9% - at the European Parliament elections in 1989 but did not win any seats. Since 1999 it has benefited from a change to a more proportional system of voting for MEPs. It picked up two MEPs in 1999 which it retained in 2004 and 2009, when it secured a record 1.3m votes.
The Green Party in England and Wales currently has 125 councillors, two MEPs and two London Assembly members.
But historically the party has fared less well in general elections, where it is hampered by the first-past-the-post voting system. In the 2005 general election, it secured 257,695 votes - just 1% of the overall national vote.
New leader Ms Lucas hopes to become MP for Brighton Pavilion - a seat currently held by Labour where the Greens came a close third in 2005, winning 22% of the vote, and have a strong presence on the local council with 13 seats.
It is also focusing its resources on former Labour Home Secretary Charles Clarke's Norwich South constituency and Lewisham Deptford in south London, also held by Labour.
Overall the party hopes that more than 300 candidates will stand in England, Wales, Scotland and Northern Ireland.
The party has made some efforts to widen its appeal beyond purely environmental issues to include creating "sustainable" jobs in areas like alternative energy and housing.
It also opposes ID cards and renewing the Trident missile system and urges radical reform of the financial system is necessary.
And, like other smaller parties, it will hope to benefit from the expenses scandal which has hurt the main parties.
The Scottish Greens were founded as the Scottish Ecology Party in 1979 as part of the main UK Ecology Party.
In 1990 the Scottish Green party became a separate party from the Greens in England and Wales, and produced its own manifesto "Towards a Green Scotland".
Although the party shares campaigns and ideas with the Greens south of border, there have been some policy differences - for example on the abandoned EU constitution.
The Scottish Green Party has also retained the structure of having two "co-convenors" instead of one leader.
The party has had some success under the proportional representation system used for the devolved Scottish Parliament. In 2003 it won seven seats in the Scottish Parliament but this dropped to two seats in 2007.