Barack Obama enters the White House with a wide-ranging agenda for change.
The BBC's Max Deveson considers which issues he is likely to tackle head on - and which he may be tempted to put to one side:
To stimulate the ailing US economy, Mr Obama wants to make the biggest investment in the country's infrastructure since President Eisenhower constructed the Interstate Highway system in the 1950s.
He proposes a countrywide road- and bridge-building programme, as well as nationwide schemes to refurbish public buildings and schools, bring them up-to-date and make them energy-efficient. By investing in projects that are already in the works, Mr Obama hopes to inject cash into the system quickly, create jobs and trigger consumer spending.
Other proposals to encourage job growth and bolster consumer spending include tax credits for firms that create jobs, tax cuts for 95% of American workers, and extended unemployment benefits.
The cost of the stimulus package has been estimated at between $700bn (£475bn) and $1 trillion.
Mr Obama has pledged to reduce insurance costs, while offering a new affordable public plan for those who do not have insurance, and he has backed up his rhetoric with appointments that signal he means business.
Tom Daschle, former Senate Majority Leader, will head the Health and Human Services Department and also act as the White House's health policy tsar. Several other key advisers also have experience of getting legislation through Congress.
These Congressional veterans will be working with a congress that is itself keen on healthcare reform - two prominent senators, Ted Kennedy and Max Baucus, are working on their own reform plans.
Mr Obama still faces a tough battle to win over insurance companies, pharmaceutical firms, doctors, and the handful of Republican Senators he needs to get a plan through the Senate - but the wind is definitely blowing in his direction.
Mr Obama has pledged to ensure that 10% of America's electricity comes from renewable sources by 2012, and 25% by 2025. He also wants an 80% cut in greenhouse gas emissions by 2050. He proposes to invest $150bn in alternative fuel over the next 10 years, and will work in 2009 to establish a "cap and trade" programme to reduce greenhouse gas emissions.
Under the scheme, similar to the EU Emission Trading Scheme, firms would be allocated emissions permits, and those who emit more than their quota would be forced to buy permits from those who emit less.
The scheme is likely to come up against fierce opposition in congress. But with large Democratic majorities in both the House of Representatives and the Senate, and with environmentalist Henry Waxman now chairing the House Energy and Commerce Committee, the odds have improved in Mr Obama's favour.
FROM IRAQ TO AFGHANISTAN
Mr Obama is proposing to redeploy troops from Iraq at a pace of one or two combat brigades a month, which would mean that withdrawal would be complete by summer 2010.
This would allow Mr Obama to fulfil his other major foreign policy pledge - to increase troop levels in Afghanistan. Military officials say some 20,000-30,000 extra troops could be sent to the country by summer 2009, doubling the existing number of troops there.
Mr Obama's opposition to the Iraq War was one of the main reasons for his initial rise to prominence. Although his national security team consists of people who largely supported the war, Mr Obama has made it clear that withdrawing troops from Iraq is still on his agenda.
Closing the controversial prison camp for terror suspects at the naval base in Guantanamo and ending the use of "enhanced interrogation techniques" are central planks of Mr Obama's security agenda.
In a recent interview with Time magazine, he said that if his administration had not "closed down Guantanamo in a responsible way, put a clear end to torture and restored a balance between the demands of our security and our constitution" within two years of taking office, then it would have failed.
If Mr Obama does close Guantanamo, then his administration will have to work out what to do with the men being held there. Many cannot be sent back to their home countries, for fear they might be tortured.
Mr Obama will also need to decide whether to continue the current much-criticised system of military tribunals, to try suspects in US criminal courts, to free them, or to develop an alternative.
ENGAGING WITH THE WORLD
Polls suggest that in contrast to his predecessor, Mr Obama is very popular in many countries - a big change, which he is likely to exploit.
He has pledged to give a major speech in the Muslim world early in 2009 - perhaps in Egypt, or in Indonesia (home to the world's largest Muslim community) where he lived for a brief period as a child.
If he can improve America's standing in the world, it will be easier for him to ask more from America's allies - to persuade Nato members to send more troops to Afghanistan, or Israel to make concessions as part of an invigorated Middle East peace process.
Mr Obama has pledged to "make progress on the Israeli-Palestinian conflict a key diplomatic priority from day one". International goodwill could be key if he is to succeed where so many of his predecessors have failed.
The biggest fight of Mr Obama's first year in office may not be over healthcare or Iraq, but over a bill to make unionisation easier.
The Employee Free Choice Act (EFCA) is one of the US trade union movement's key demands. It would force employers to recognise a union if a majority of employees vote for it, either by public "card check" voting or (if 30% of the workforce ask for it) by secret ballot.
Employers' groups are opposed to the legislation - they fear that a unionised workforce will increase labour costs, and argue that the bill will deny workers their right to a secret ballot.
Mr Obama says he supports the legislation, but there will be plenty of opposition to it in Congress, so he may prefer to expend his political capital on his top priorities - healthcare and climate change. However, if the unions demand EFCA in return for backing Mr Obama on the rest of his domestic agenda, he may have to think again.
ON THE BACK BURNER
There are a number of proposals in Mr Obama's in-tray for which he professes support, but which could jeopardise the rest of his agenda. The EFCA is just one of these.
In Bill Clinton's case, a bid to overturn the ban on gay people in the armed forces weakened his ability to deliver healthcare reform. So it's likely that Mr Obama will move cautiously on federal laws allowing civil unions for same-sex couples, and steps to replace Clinton's compromise on gays in the military (known as "Don't Ask, Don't Tell").
Another example is immigration reform. Like George Bush, Mr Obama wants to create paths to citizenship for existing illegal immigrants, while strengthening border security. Mr Bush's failure may make him think twice.
If Mr Obama is able to get some early legislative wins under his belt, however, then federally-recognised civil unions for same-sex couples and immigration reform could well be on his agenda before too long.
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