The second term begins in circumstances different in many ways from the first, but familiar in others.
The economy, then in the grip of a fierce and deepening recession, is now recovering slowly as unemployment recedes and stocks flirt with five-year highs. The health care legislation that Obama urged Congress to enact in his first inaugural address is the law of the land, courtesy of a split ruling by the Supreme Court.
Al-Qaida leader Osama bin Laden is dead at the hands of U.S. special operations forces. But the organization he inspired is far from moribund, as demonstrated by the just-ended kidnapping episode Saturday at an Algerian natural gas complex that, according to the Algerian government, left at least 23 hostages dead. The U.S. on Friday acknowledged one American death.
When Obama took office in 2009, his Democratic allies held control of Congress.
Now, divided government rules, and Republicans who control the House lead the way in insisting the administration agree to spending cuts that will reduce soaring federal deficits. Obama has said he is ready to compromise on that.
At the outset of a second term, he also wants Congress to overhaul the nation's immigration laws and take steps to reduce gun violence in the wake of the shooting last month in Newtown., Conn., that left 20 elementary school children dead.
Yet for once, politics seemed to edge ever so slightly into the background in the most political of cities.
Former first daughter Chelsea Clinton headlined a National Day of Service gathering under a tent on the National Mall, where she said she had been inspired by her grandmother, as well as her famous parents. She urged her audience to become part of a "chain of service" by helping the less fortunate.
Biden and his wife, Jill, spent time at an armory pitching in as volunteers packed 100,000 care kits for deployed members of the military, wounded warriors, veterans and first responders.
Biden credited former President George H. W. Bush, a Republican, for starting the "Points of Light" program, which was a sponsor of the event. He said service was an antidote to "the coarsening of our culture. We've got to get back to reaching out to people."
In the evening, Mrs. Obama and Mrs. Biden hosted the Kids' Inaugural Concert, an event paying special tribute to military spouses and children.
Families invited to the event cheered Usher as he sang his hit "Yeah!" Katy Perry, who donned star and stripes for her song "Firework," told the audience, "I'm very proud to be here ... and to see the Obamas and the Bidens here for four more years."
Associated Press writers Darlene Superville, Laurie Kellman, Frederic J. Frommer and Jocelyn Noveck contributed to this report.