A bill known as the Respect for Marriage Act is working its way through Congress and would repeal DOMA.

That law does not prohibit states from allowing same-sex marriages, but it also does not force states to recognize them from other states. Most of the current plaintiffs are federal workers, who are not allowed to add their spouses to health care plans, and other benefits.

Many other states, including New Jersey, Illinois, Delaware, Rhode Island and Hawaii, have legalized domestic partnerships and civil unions for such couples -- a step designed in most cases to provide the same rights of marriage under state law.

But other states have passed laws or state constitutional amendments banning such marriages. California's Proposition 8 is the only such referendum that revoked the right after lawmakers and the state courts previously allowed it. That makes it a somewhat unique legal case for review by the justices.

The DOMA cases are U.S. v. Windsor (12-307) and Bipartisan Legal Advisory Group of the U.S. House of Representatives v. Windsor (12-785).