Last Updated 2:18 p.m. ET
Elena Kagan was sworn in as the 112th Supreme Court Justice today, and the fourth to sit on the nation's highest court.
Kagan will be formally installed at the start of the court's next term, on the first Monday in October.
Kagan was actually sworn in twice today by Chief Justice John Roberts. She recited the oath as prescribed by the Constitution during a private ceremony in a conference room at the Court; only her family was present.
Roberts then administered a judicial oath, with Kagan's family and friends and reporters present.
Roberts then congratulated Kagan, to boisterous applause.
The former Harvard Law School dean was most recently solicitor general.
Republicans criticized her as a political liberal after President Obama nominated her to replace retiring Justice John Paul Stevens, who turned 90 this year. But the Senate confirmed her this past week on a vote of 63-to-37.
At 50, Kagan is now the youngest member of the Court. But she isn't expected to alter the ideological balance, where Stevens was considered a leader of the liberals.
Speaking at the White House on Friday, Kagan thanked Mr. Obama for trusting her enough to nominate her to the Supreme Court.
She pledged to fulfill an "obligation to uphold the rights and liberties afforded by our remarkable Constitution" and "to provide what the inscription on the Supreme Court building promises: equal justice under law."