Next week, the US Supreme Court will take up the issue of gay marriage, which is legal in nine states and the capital Washington but banned under the federal Defense of Marriage Act of 1996.
Here is a rundown of the current statutes governing same-sex unions in the United States, where recent polls show public opinion has swung toward favoring the legalization of gay marriage.
DOMA defines marriage as the union of one man and one woman, granting legal recognition only to heterosexual marriages.
US President Barack Obama came out in support of gay marriage on May 9, 2012 and in January of this year delivered the first-ever inaugural address specifically calling for equal rights for gays and lesbians.
Former president Bill Clinton, who signed DOMA into law in 1996, earlier this month called for the Supreme Court to overturn it, saying the law was "discriminatory."
STATES WHERE SAME-SEX MARRIAGE IS LEGAL
The US Northeast led the way in legalizing gay marriage at the state level, with Massachusetts doing so in 2004, followed by Connecticut in 2008, Vermont in 2009, New Hampshire in 2010 and New York in 2011.
The Midwestern state of Iowa legalized gay marriage in April 2009, followed by the District of Columbia home to the US capital Washington in December of that year. Two Indian tribes have also legalized gay marriage.
Maine, Maryland and Washington state approved gay marriage by referendum during last November's election.
STATES WHERE GAY MARRIAGE IS OUTLAWED
Thirty-one states have adopted measures outlawing gay marriage. Thirty of those states have passed amendments to their constitutions defining marriage as between one man and one woman, including North Carolina, which approved its amendment last May. Hawaii banned gay marriage in 1998.
In another 10 states, same-sex marriage is not possible due to the family code or the absence of authorizing legislation.
STATES ALLOWING CIVIL UNIONS
Nine states -- California, Colorado, Delaware, Hawaii, Illinois, Nevada, New Jersey, Oregon and Rhode Island -- allow civil unions, which entitle gay couples to the same legal rights as married couples. Colorado's law will go into effect in May.
California's high court ruled in 2008 that same-sex couples have the right to marry under the state's constitution, but voters approved a referendum outlawing gay marriage six months later.
On March 26 and 27, the Supreme Court will consider separate cases on the legality of California's ban -- the so-called Proposition 8 -- and DOMA.