Millions of Americans are to receive health insurance cover for the first time as President Obama's flagship healthcare reforms come into effect.
The reforms are part of the president's aim to ensure affordable healthcare is available to everyone.
But the policy is controversial and the roll-out of the new system has been beset with problems.
Some religious-affiliated groups won a last-minute reprieve from being forced to provide birth control cover.
Supreme Court Justice Sonia Sotomayor temporarily blocked the government from forcing such groups to offer health insurance that would include contraception.
She acted at the request of a group of Catholic nuns in Colorado, the Little Sisters of the Poor, who had earlier lost their request for a preliminary injunction at an appeals court in Denver.
They had argued that the ruling conflicted with the Catholic Church's stance against the use of contraceptives.
Under the Affordable Care Act, popularly known as Obamacare, it is now compulsory for people to have health cover - either provided for by their employer or by buying one of the private health plans now on offer.
Those who cannot afford it will get help, but those without any insurance will be fined.
As of 1 January 2014, health insurance companies are also no longer able to deny coverage to people with pre-existing conditions.
More than 2.1 million people have enrolled so far for private health plans - short of the government's original target.
But the phased roll-out of the new law has suffered a number of difficulties.
The federal website offering the new health plans was plagued with technical glitches when it was launched in October. There were long sign-in wait times, log-in difficulties, insurance account creation problems, slow page loads and outages.
Insurance companies have also announced the cancellation of millions of policies, saying they did not meet the law's minimum requirements.
This came despite President Obama's promise that people would not be forced to move from plans they were happy with.
Mr Obama's approval ratings fell in the wake of the problems, but the White House says things have been fixed, the BBC's Rajini Vaidyanathan reports from Washington.