One of those cases is a challenge to the Defense of Marriage Act, which says the federal government doesn't recognize same-sex marriages. Many original backers of DOMA have renounced it, and gay rights activists are confident the high court will kill the 1996 law.

The other case is more uncertain. Today, the high court will tackle a dispute over California's Proposition 8, a voter-approved ban on gay marriage in the Golden State.

A judge and a federal appeals court ruled Prop 8 was unconstitutional, and a coalition of anti-gay marriage advocates have asked the Supreme Court to revive the law.