The Labour Party was born out of the trade union movement in 1900 with the aim of giving a political voice to the working classes.
Ramsay Macdonald was the first Labour premier
The creation of a national government during the First World War gave the party its first ministers and a taste of political power.
After the war, the decline of the Liberal Party saw Labour becoming the main opposition party, but although twice in power, Labour governments were short lived.
It was the Second World War that changed the party's fortunes.
In the coalition government that took over in 1940, the Labour leader Clement Attlee became deputy to Winston Churchill. The Beveridge Report, prepared during the war years, created the vision of a better society, and heralded the social changes that would follow in peacetime.
But the scale of the Labour landslide at the 1945 election came as a shock even to the party's own supporters.
Attlee's premiership saw the creation of the NHS
Post-war Britain saw the creation of the welfare state and the nationalisation of key industries like coal, steel and the railways. But disappointment followed; the Tories won the 1951 election, and an era of prosperity helped to keep Labour out of power for 13 years.
In the 1960s, under the leadership of Harold Wilson, a rejuvenated Labour Party clawed its way back to power, first with a tiny majority in 1964, and then a convincing victory in 1966.
But the new administration was bedevilled by economic problems, leading to the devaluation of the pound and in
1970, the Conservatives returned to power.
By 1974 Labour was back, winning two elections in the same year, but facing new problems.
Harold Wilson was prime minister in the 1960s and 1970s
After the surprise resignation of Harold Wilson as party leader, Jim Callaghan became prime minister. He quickly found himself presiding over an economic crisis, requiring a hefty international loan and cuts in public spending.
But the final straw for many voters was the sight of the country being paralysed by union disputes during the "winter of discontent."
At the 1979 election, Labour was voted out and Margaret Thatcher moved into Downing Street. It marked the start of 18 years in opposition for Labour, and one of the bleakest periods in the party's history.
The Conservatives were to win four elections in a row, three under Mrs Thatcher and another under John Major.
During Labour's wilderness years, the party leadership passed first to the left-winger Michael Foot.
A group of former ministers on the right of the party, the "gang of four", left Labour to found the Social Democratic Party.
James Callaghan's period in power was beset by industrial strife
Labour's manifesto for the 1983 election was later described as "the longest suicide note in history". Mr Foot was followed by Neil Kinnock, who faced the challenge of the Militant Tendency as he set about modernising the party.
But while Labour was slowly edging back towards power, Mr Kinnock was unable to steer the party to the ultimate victory.
He was succeeded by John Smith, a highly respected figure in Parliament, widely seen as the prime minister in waiting. His sudden death in 1994, after a heart attack, was a shattering blow for the party.
In the contest that followed, Tony Blair was elected leader and the progress of modernisation begun by Neil Kinnock continued with a vengeance.
The architects of "New Labour" were pitted against the forces of tradition, with their attachment to socialism - regarded by the leadership as an electoral liability.
In a symbolic break with the past, the Clause 4 commitment to nationalisation was torn up.
Neil Kinnock saw off the Militant Tendency
Re-positioned as a party of the centre-left, Labour approached the 1997 election at a time when the Conservative government appeared to be running out of ideas. Individual Tory MPs were also tainted by allegations of "sleaze".
Labour's election manifesto presented the party as one offering a "new politics" that would mean an end to old divisions between Left and Right. The result was a landslide victory that made Tony Blair Prime Minister.
Labour had committed itself to modernise Britain and build a fairer society, but Mr Blair knew the electorate needed reassuring that past mistakes would not be repeated - not least when it came to the economy.
The unions would be treated fairly but without any favours. And Labour would welcome business as a partner. In government, the party clearly intended to hold the middle ground.
The ambition was to be the first Labour government to win a second full term - and that they did with apparent ease.
Tony Blair became Labour leader in 1994
Setting out his ambition for that second term Mr Blair said: "It is time for a second phase of New Labour, defined less by reference to the old Labour Party, than by an agenda for the country, radical but firmly in the centre ground", he said.
"How far we are from a society of true equal opportunity is a measure of how far a radical New Labour government has to go."
Four years on Mr Blair is now seeking a full third term which if he succeeds would see him beat Margaret Thatcher's 11 and a half years in Number 10.
He will want to spend that third term overhauling the public services having spent the first two highlighting Labour's record on the economy, citing low interest rates, low inflation, steady growth and high employment.
But the prime minister knows that despite the prosperity enjoyed by voters in recent years he could face a backlash at the polls over his decision to join the US in invading Iraq.