March 23, 2010: President Obama signs the Affordable Care Act into law. The broadest change to the U.S. health-care system in decades, it is intended to provide coverage to millions of uninsured Americans through new private insurance marketplaces, called exchanges, and an expansion of Medicaid.
Spring 2010: An office to build the insurance exchanges and other parts of the law is created within the office of Health and Human Services Secretary Kathleen Sebelius.
July 2010: HealthCare.gov launches for the first time, without the enrollment function.
Sept. 23, 2010: The law bars insurance companies from refusing to cover children with preexisting conditions, from dropping people who get sick and from placing lifetime limits on coverage. It allows people to stay on parental health plans until age 26.
Jan. 1, 2011: Insurance companies are required to spend 80-85 percent of premiums on medical care and health-care quality improvement; as a result, many Americans begin to receive annual refunds. Preventive care becomes free under Medicare, the federal insurance program for people 65 and older.
Early 2011: The Centers for Medicare and Medicaid Services — a federal agency spread among locations in the Washington, D.C., area — takes over responsibility for building the marketplaces from the unit that had reported to Sebelius. As a result, the work of building the federal exchanges becomes fragmented.
February 2011: The White House official overseeing work on the law, Nancy-Ann DeParle, is promoted and begins to spend only a small fraction of her time on it. Her successor is Jeanne Lambrew, who had worked on health reform under Bill Clinton.
Aug. 12, 2011: The 11th Circuit Court of Appeals rules that parts of the law are unconstitutional.
Nov. 8, 2011: The U.S. Court of Appeals in Washington rules that the law is constitutional.
Nov. 14, 2011: The Supreme Court agrees to hear a challenge to the law after 26 states, led by Florida, petitioned the court.
June 28, 2012: The Supreme Court rules that the individual mandate portion of the health care law may be upheld within Congress’ power under the taxing clause.
Oct. 1, 2013: People begin shopping for health insurance on HealthCare.gov, but the site crashes after a relatively small number of insurance-seekers try to create accounts.
Jan. 1, 2014: Coverage through Affordable Insurance Exchange takes effect. Most individuals are required to have insurance or face a penalty.
Jan. 1, 2015: Physician payments are tied to the quality of their care. Employers with more than 50 workers are required to provide benefits.