The US Supreme Court will this week begin a new chapter in its review of discrimination based on sexual orientation.
Nine Supreme Court justices will consider the legality of a California ballot initiative that limits marriage to opposite-sex couples.
It has been a decade since the court last took up a gay rights dispute - a 2003 case in Texas was closely fought and produced strong dissenting opinions.
Tuesday will be the first of two days of arguments on the issue. On Wednesday, the court will consider the 1996 federal Defense of Marriage Act (DOMA), which limits the definition of marriage to opposite-sex couples.
Rulings in both cases are expected by the end of June.
The cases come before the high court at a time when more states have legalised gay marriage. Last year three more - Maryland, Maine and Washington - did so, bringing the total to nine plus the District of Columbia.
A timeline of gay rights legislation under President Obama
- 2008 - California gay marriages become legal when the California Supreme Court strikes down the Proposition 22 ban.
- 2009 - Iowa State Supreme Court legalizes same-sex marriage. It is still the only Midwest state that allows gay marriage.
- 2011 - Barack Obama's administration ends "don't ask don't tell policy," allowing gays to serve openly in the military.
- 2012 - North Carolina approves a constitutional amendment to ban gay marriage in May. In November, Maine, Maryland and Washington become the first states where voters approve same-sex marriage, and Minnesota rejects a new ban.
- 2012 - Barack Obama endorses same-sex marriage.