One of the central provisions of President Barack Obama and the Democrats' healthcare reform law, known popularly as "Obamacare", took effect on 1 October.
But the rollout has been a White House public relations nightmare, with a federal website bedevilled by errors and millions of Americans' existing policies cancelled, contrary to Mr Obama's promise.
The Patient Protection and Affordable Care Act, signed by Mr Obama in 2010, is the largest overhaul of the US healthcare system since the 1960s.
What does the law do?
Its aim is simple: to extend health insurance coverage to some of the estimated 15% of the US population who lack it. Those people receive no coverage from their employers and are not covered by US health programmes for the poor and the elderly.
To achieve this, the law requires all Americans to have health insurance, but offers subsidies to make coverage more affordable and aims to reduce the cost of insurance by bringing younger, healthier people into the health insurance system.
It also requires businesses with more than 50 full-time employees to offer health coverage, although this provision was delayed until 2015 to allow more time for compliance.
The law creates marketplaces - with websites akin to online travel and shopping sites - where individuals can compare prices as they shop for coverage.
In addition, the law bans insurance companies from denying health coverage to people with pre-existing health conditions, allows young people to remain on their parents' plans until age 26, and expands eligibility for the government-run Medicaid health programme for the poor.
The law aims eventually to slow the growth of US healthcare spending, which is the highest in the world.
What went wrong?
The phased rollout of the law has been bedeviled by setbacks, including:
July 2013: The mandate that businesses with over 50 workers must provide insurance is delayed a year until 2015
Oct 23: The deadline for individuals to avoid penalties is pushed back six weeks to March 2014
Oct 30: Mr Obama acknowledges "no excuse" for healthcare.gov website meltdown, which his health secretary concedes is a "debacle"
Nov 14: Mr Obama announces insurers can keep customers on existing plans for another year, amid fury at his broken promise
Nov 22: Enrolment deadline for individuals is pushed back a week in December
Nov 26: Spanish-language sign-up tool is postponed until December
Nov 27: Online insurance enrolment for small businesses is delayed for a year
Feb 4 2014: Congressional budget analysts predict the health law will cut the US workforce by equivalent of 2.3 million workers by 2021
Feb 10: The mandate that businesses with over 50 workers must provide insurance is delayed for another year, until 2016
What happened to the website?
Healthcare.gov is a federal website that serves as a marketplace where individuals in 36 states can compare private insurance plans, learn about available public subsidies, and sign up for a coverage. The remaining 14 states and Washington DC have set up their own websites.
The federal portal was plagued with technical glitches from its rollout on 1 October, including long sign-in wait times, log-in difficulties, insurance account creation problems, slow page loads and outages.
Exchanges run by individual states did not appear to have the same problems. Enrolment figures for the first month were a fifth of what the Obama administration initially projected.
The White House has promised a "smooth experience" for the "vast majority" of website users by the end of November.
Why were some Americans' plans cancelled?
In autumn 2013, insurance companies began announcing the cancellation of millions of policies that do not meet the law's minimum requirements, including coverage of a range of preventive and emergency healthcare.
Millions of Americans with individual plans were affected, despite Mr Obama's oft-repeated promise that those who liked their plans "could keep them".
Mr Obama apologised to those whose policies had been cancelled and then announced he would be giving a one-year reprieve from the cancellations. But insurance firms have questioned whether the proposed fix is workable.
Why do conservatives oppose the law?
Republicans say the law imposes too many costs on business, with many describing it as a "job killer". They have also decried it as an unwarranted intrusion into the affairs of private businesses and individuals.
The party and a veritable industry of conservative think tanks and advocacy groups have fought the law since Mr Obama first proposed it in 2009 at the start of his first term in office.
After the law was passed in 2010, Republicans launched a legal challenge, which ended in June 2012 when the US Supreme Court declared it constitutional. It was also a central issue in the 2012 presidential election, when Mr Obama won a second term in office.
Meanwhile, the House of Representatives, controlled by the Republicans, has taken dozens of symbolic votes to repeal the law and forced a partial government shutdown over the issue. Republicans in state capitals have also sought to undermine it in various ways.
Democrats say Republicans are politically motivated to attack Mr Obama's flagship domestic achievement in order to weaken him.
When do its elements take effect?
The law takes effect in several stages:
2010: Insurers were banned from denying coverage to children with pre-existing conditions, adults under 26 years of age were allowed to remain on their parents' plans, and insurers were barred from cancelling coverage in some circumstances. Insurers were forbidden from imposing lifetime limits on health coverage.
2011: The law expanded access to preventative health services, and offered drug discounts for people in the Medicare programme for pensioners.
2012: Provisions to encourage healthcare providers to lower costs took effect.
October 2013: Consumer health insurance marketplaces go online, and is plagued by snags and glitches.
2014: People without health coverage from the government or their employers will be required to purchase it. The government Medicaid health programme for the poor will be expanded to cover more people, tax credits will subsidise individual and family purchases of health policies, and insurers will be barred from denying coverage to those with pre-existing health conditions.