Tony Blair surged to power in 1997 with an historic general election victory which ousted the Conservatives after 18 years in office.
Blair says appetite for power is 'undiminished'
And on 2 August 2003 he became Labour's longest continuously-serving prime minister - passing Clement Attlee's record of six years and 92 days.
The 1997 triumph was Labour's greatest ever electoral victory: a huge Commons majority of 180 giving Mr Blair the mandate to make radical changes; to the National Health Service, education and social security.
Mr Blair's reforming zeal, which he had initially used to transform the Labour Party, became immediately apparent.
The government gave the Bank of England the power to set interest rates, Scotland and Wales voted for devolution, work began on reforming the House of Lords and Londoners decided to elect their own mayor and assembly.
In Northern Ireland, the 1998 Good Friday Agreement, reached after days of seemingly endless negotiations, held out the prospect of a new era of peace and prosperity.
But, even in the early months of his first administration, criticism grew of Mr Blair's reliance on media management, or "spin".
Behind the scenes, key advisors like the Downing Street press secretary, Alastair Campbell, and another architect of New Labour, Peter Mandelson, wielded huge influence, both over the government's message and the manner in which it was presented.
The dichotomy at the heart of Mr Blair's approach - wanting to disperse power, yet to ensure that his beliefs were shared by those who received it - brought its own particular problems.
He made disastrous interventions in the selection of candidates in Wales and London, leading to accusations that he was a "control freak".
And the party that had once accused the Tories of sleaze now faced media attacks itself, most notably over a £1m donation to the party from the boss of Formula One motor racing, Bernie Ecclestone.
Spinmeister: Alastair Campbell with Tony Blair
Away from domestic politics, Mr Blair forged a close friendship with his ideological soul-mate, Bill Clinton. Together, they pursued a robust policy against the Serbian president, Slobodan Milosevic.
In 1999, Royal Air Force warplanes participated in air-strikes against Serbia to force its troops out of the province of Kosovo.
Tony Blair has always been an enthusiastic supporter of the European Union, and signed the UK up to its Social Charter - guaranteeing workers' rights - as soon as he entered office.
However, he has always been more cautious on the question of UK participation in the European single currency, the euro, preferring the "wait and see" approach of the previous UK prime minister, John Major.
That was most recently confirmed when the government put off a decision on whether to call a referendum on joining the currency until 2004.
The general election of 2001, despite the Labour government investing more than any before in education and health, was dominated by widespread dissatisfaction over the slow pace of reform in public services.
Lengthening hospital waiting lists, rising levels of crime and the lack of an effective opposition contributed to one of the lowest-key elections for years.
Blair is berated by Sharon Storer (l) in 2001
During the run-up to the election, Mr Blair was confronted by Sharon Storer, whose partner was being treated for cancer in a local hospital.
She berated the prime minister for the poor quality of care. He, for once, looked embarrassed and out of touch.
With a divided and unpopular Tory party, the general election result was never in doubt and Mr Blair was returned to Downing Street with a huge, if slightly smaller, majority.
But the questions over the speed of reform continued to dog the prime minister even as international events overshadowed the home front.
The 11 September attacks on the United States had an enervating effect on Tony Blair.
Thrust onto the world stage, he played his role with aplomb from the start, giving critical support and advice to the new US president, George W Bush.
Yet, even these terrible events were somewhat overshadowed when Jo Moore, a New Labour spin-doctor, suggested in an internal email that the day of the attacks would be "a good day to bury news".
She apologised and eventually resigned, as did her boss, the transport secretary, Stephen Byers.
He joined others ministers like Peter Mandelson, Keith Vaz and Geoffrey Robinson, who have been forced out of Mr Blair's administrations.
Spin, and accusations of cronyism - Mr Blair has made two close personal friends, Lords Falconer and Irvine, into ministers - have followed the prime minister throughout his time in office, often clouding his achievements.
Over Iraq, Tony Blair's conviction, that Saddam Hussein should be disarmed, pitted him against many Labour voters and supporters.
United: Tony Blair and George W Bush
Europeans, who once held him to be a positive believer in the European dream, are now no longer so sure. Blair the friend of Brussels is now Transatlantic Tony.
Old Labour supporters, who never signed up to his vision of a "third way", independent of unreconstructed socialism and old-fashioned capitalism, saw an opportunity to make one last stand.
In standing alongside President Bush, Mr Blair has managed to alienate many traditional Labour supporters who remain deeply suspicious of the world's only remaining superpower, and in particular of the current occupant of the White House.
That distrust is also shared by many in Europe and the prime minister faced the daunting - some say impossible - task of trying to bridge the gap between the views of 'old Europe', led by France and Germany, and the unflinching position of the American administration.
Mr Blair's supporters argued that he was not meekly following the American line, but instead remained true to a position he has held for many years, that if Saddam Hussein would not disarm peacefully then force would have to be used.
In the event, action against Iraq went ahead despite massive Commons rebellions and the resignation of two cabinet ministers - one before the conflict and one afterwards.
Mr Blair, in a widely admired speech to MPs, won the day with his insistence that without military action, Saddam would continue to defy the international community.
Victory: The Blairs enter No 10 in 1997
But while the war was won, the much-expected "Baghdad bounce" which Downing Street hoped would boost Mr Blair's popularity to new heights failed to materialise.
The weapons programmes in Iraq which the prime minister warned about have, as yet, not been found. There has been disquiet about an alleged lack of planning for post-war Iraq.
Questions have been raised about the intelligence upon which the UK went to war - prompting a bitter and divisive row between the government, and particularly media chief Alastair Campbell, and the BBC.
And the death of Iraq weapons expert Dr David Kelly, who apparently killed himself after giving evidence to MPs investigating the way the case for war was made, looms over the political agenda.
An inquiry into the death of the scientist - who was the source of BBC stories questioning the way the case for war was presented - is dominating the summer months at a time when the Blair administration might otherwise have been celebrating an historic anniversary as the longest-serving Labour government.
The prime minister says his appetite for the job ahead of him - which includes winning the trust of those doubting both the merits of the Iraq war and proving his commitment to public service reform - remains undiminished.
The coming months will test that resolve in the extreme.