Election profile: The Labour Party
At the 2015 general election Labour will seek to bring an end to five years in opposition after rebranding as the "one-nation" party under its youngest ever leader, Ed Miliband.
Born out of the trade union movement in 1900, the Labour Party grew to eclipse the Liberals over the first half of the 20th Century as the Conservatives' main opposition - culminating in its landslide victory in 1945.
The post-war Labour government ushered in changes that would become central to how the party saw itself, creating the welfare state, founding the NHS and nationalising key industries such as coal, steel and the railways.
Its leader Harold Wilson first came to power in the 1960s on a modernising platform but his administration was plagued by economic problems, which led to the devaluation of the pound.
He led Labour back to victory in 1974 - but this time he faced divisions within the party over the common market and soaring rates of inflation.
His surprise resignation saw Jim Callaghan become leader - but his time in office became associated with an economic crisis, requiring a hefty international loan and cuts in public spending.
Unions went on strike over curbs to public sector pay during the "winter of discontent" - setting the scene for Margaret Thatcher's election in 1979 and the start of 18 years in opposition for Labour.
During that time Labour was led by left-winger Michael Foot, whose manifesto for the 1983 election was later described as "the longest suicide note in history".
A new phase of modernisation was begun by Neil Kinnock, continued by John Smith - who died of a heart attack in 1994 - and ultimately converted into electoral success by Tony Blair.
The birth of "New Labour" under Blair involved the scrapping of the manifesto's Clause 4 commitment to nationalisation and a new friendliness towards business and the private sector.
New Labour was proud of its record on investing in schools, childcare and equality legislation - but there was also public anger over the Iraq war in 2003.
Mr Blair stood down in 2007 and was replaced by his chancellor, Gordon Brown, without a contest.
Then, after years of economic boom, the credit crunch hit, forcing Mr Brown to bail out the banking sector and ratchet up Britain's deficit.
The MPs' expenses row in 2009 hit all the major parties, but may have helped fuel a desire for change which contributed to Labour's defeat at the 2010 election.
Ed Miliband became leader after winning an extremely narrow and unexpected victory over his brother and former Foreign Secretary David Miliband.
Despite being seen as to the left of Tony Blair, under Mr Miliband's watch the party has courted the middle ground.
He said at the 2012 party conference he believed in "the spirit of one nation" which he characterised as "a country where everyone has a stake" and "a country where prosperity is fairly shared".
We know so far that includes a freeze on energy bills, a mansion tax, renewed efforts to curb tax evasion and reform of the EU, most likely without a referendum.
The Labour Party is also, naturally, defined to some extent by what it opposes: prominent on its list of objections to coalition policy is the housing benefit changes it calls the bedroom tax, which it has vowed to scrap.
What we don't know is whether this version of Labour will persuade voters to put the party back in the driving seat.