Barack Obama was elected on a message of change. Now he is in office, change is expected both in foreign and domestic policy. Here the BBC's team in Washington tracks developments in the first 100 days of the Obama presidency.
FRIDAY 17 APRIL - DAY 88
1650 EDT Carbon revolution
BBC North America Editor Justin Webb
: The Environmental Protection Agency has ruled that carbon dioxide is a health risk, a ruling that could lead to regulation of emissions around the nation and may encourage Congress to pass far-reaching legislation on greenhouse gases.
This is another of those hand-brake turns from the Bush route to the new course set by Obama. Not an unexpected change of direction, but nonetheless hugely significant; a policy change that will alter America forever.
The Environmental Protection Agency's announcement contains the simple assertion that greenhouse gas pollution is a serious problem now and for future generations. The gases, the agency says, contribute to air pollution and may endanger public health or welfare.
Back in April 2007, the Supreme Court ruled that the EPA's previous refusal to consider regulating emissions of carbon dioxide was unlawful, but the justices did not force the agency to begin regulation, only to come up with a view on whether or not carbon was harmful.
The agency decided it was, but the Bush administration took no further action.
Now, the agency - under the new administration - is in the driving seat.
This announcement is the first step towards a huge new nationwide system of regulation.
There will be a period of public consultation first and there is bound to be intense opposition, with opponents claiming already that shopping centres and small businesses will be subjected to extra costs at the very moment when they can least afford it.
The solution will probably be congressional action that addresses the red tape issue. There is plenty of argument still to be had, but America is now heading in the direction of European-style awareness of carbon emissions and efforts to limit them.
THURSDAY 16 APRIL - DAY 87
1555 EDT 'Torture' memos released
: The Justice Department has released four much-anticipated Bush-era memos written by staff at the Office of Legal Counsel in 2002 and 2005. The documents gave legal authorisation for harsh interrogation techniques regarded by many as torture.
Civil liberties groups had long sought to see the contents of the memos, but their publication had been the subject of fierce debate within the administration. Many in the CIA had feared that full disclosure would open agents up to the threat of legal action.
Before publication, civil rights campaigners had warned President Obama that a failure to publish the memos in full would leave his reputation for transparency in tatters.
Well, President Obama
opted to publish the memos in full (virtually - the names of the CIA interrogators have been redacted). But he has also given an assurance to CIA employees involved in the interrogation programme that they will not be prosecuted.
Campaigners will be pleased that the memos have been published - but how will they react to the news that the interrogators have been given immunity?
1241 EDT Trainspotting
: America's love affair with the railways ended a long time ago. It was the building of railroads that drove development across the United States in the 19th Century.
But the train has long been overtaken by the car and the plane. President Obama says that can no longer continue.
He's been giving details of a $13bn plan to overhaul America's rail network. The plan will aim to build 10 high-speed rail lines up to 1,000km long.
Mr Obama said America lagged way behind other countries in terms of high-speed rail. His plan would create tens of thousands of jobs, but it could be many years before it's completed.
He made the announcement shortly before FLYING down to Mexico for talks with the Mexican President...
WEDNESDAY 15 APRIL - DAY 86
1900 EDT Obama's Tax day
: As Kevin points out below, today was the deadline for Americans to send in their tax returns, and the president had to pay his taxes, just like everybody else. He made his tax return public, however.
2008 was a pretty good year for Mr Obama, financially - he and his wife's combined income was $2.7m (£1.8m) - although the total was lower than the $4.2m they made in 2007.
The earnings mostly came from sales of Mr Obama's best-selling books, Dreams From My Father and The Audacity of Hope. The books brought in $2.5m in royalties last year. Mr Obama earned $139,204 as a US senator, while his wife made $62,709 in her role as a manager at the University of Chicago Hospitals.
The Obamas paid $855,323 in federal taxes on the income, and $78,765 in taxes to the state of Illinois.
The tax return also reveals that the Obamas donated $172,050 to charity last year, including $25,000 to both the CARE international relief agency and the United Negro College Fund.
Unlike in 2007, the First Couple gave no money to the Trinity United Church of Christ, the church of which Mr Obama was a long-standing member until he left last year in an attempt to distance himself from its controversial pastor, Jeremiah Wright.
1850 EDT Partying against Obama
: It is tax day in the United States, which seemed the obvious moment for opponents of Barack Obama to take to the streets to register their displeasure at the spending plans he's outlined.
The protests are called "tea-parties", after the Boston Tea Party of 1773, but the modern versions do not quite have that regime-shaking intensity about them...
TUESDAY 14 APRIL - DAY 85
1721 EDT White House dog show
The new First Dog, Bo, takes his owner for a walk on the White House lawn
1404 EDT Obama's professorial economic speech
: For Barack Obama, the president who campaigned on the hope of change, every speech on the economy has to balance the optimism that got him elected with a realism that reflects the grim times through which he is leading his country.
His latest effort was long and rather professorial in tone - a lengthy dissertation on how the sudden bursting of the property bubble triggered a crisis among Wall Street bankers whose greed and stupidity (they're my words, rather than the carefully chosen words of the president) led them to invest in the ballooning housing market.
It was interesting stuff, even if Mr Obama does have a tendency to sound like someone teaching a basic economics course at an adult education college. And it was a ringing defence of what history will remember as his big idea - spending big on strategic problems like education and renewable energy with borrowed money in the depths of recession.
The bottom line? There are "glimmers of hope" but it's clear that 2009 is going to continue to be tough, with more job losses and more home foreclosures too.
This is not quite as gloomy as the talk in the first days of the Obama presidency about how we were in the midst of the worst crisis since the Great Depression, but it certainly puts into context the talk we've heard in recent weeks about how the recession might end in September.
MONDAY 13 APRIL - DAY 84
22:00 EDT Obama announces change in policy towards Cuba - with limitations
Once the CIA tried to bring down the Castro regime through dirty tricks - like planting explosives in Fidel's cigars.
The Obama administration, though, is pursuing a very different policy. It's trying to smother the communist island with cash and love.
President Obama allowed his press secretary to announce his "major" change in US policy on Cuba.
One can read too much in the choreography of the White House. But it is easy to find reasons why the president may have avoided making the announcement himself.
US policy on Cuba is highly controversial - and has been for more than 50 years.
There are around 1.5 million Cuban Americans and probably as many opinions on how to improve relations. This is not a problem that can be fixed overnight.
changes announced by the Obama administration
are important - but modest.
The reaction to them has been mixed.
Hardline critics of the Castro regime have condemned the easing of restrictions.
Lincoln and Mario Diaz-Balart - both Cuban Americans, and both Republican Congressmen - have called it a "serious mistake", one that'll provide succour to Cuba's ruthless dictators.
But most seem to welcome the news. It will, after all, make life easier and more humane.
Who can seriously condemn the act of allowing families divided by politics the chance to meet more often?
Who can really criticise relatives in a richer country wanting the help those family members living across the ocean who have to survive on less than $20 a month?
President Obama has in effect taken the easiest decisions and left the really difficult issues for later - how to deal with Cuba's leaders and what to do about the US trade embargo.
FRIDAY 10 APRIL - DAY 81
1731 EDT Obama sees "progress" on economy, but keeps mum on first puppy
President Obama expressed optimism about the state of the US economy today
, after holding a meeting with Fed Chair Ben Bernanke and FDIC Chair Sheila Bair.
He talked about the "very significant pick-up in [mortgage] refinancings", and said that he and his team "feel very good about the progress that we're making in unlocking lending".
"What you're starting to see is glimmers of hope across the economy," he added.
He was less forthcoming on the subject of the much-anticipated new White House dog, however.
"Oh, man, now, that's top secret," he joked, when a journalist asked when the dog would be unveiled.
1004 EDT The first Seder
President Obama hosts what is thought to be the first-ever Seder - a celebratory meal held to mark the beginning of the Jewish festival of Passover - at the White House