The US Supreme Court
The Defense of Marriage Act (Doma), which was signed by President Bill Clinton in 1996, has already been overturned by four federal courts and two courts of appeal, reports the BBC.
The Act defines marriage as a union between a man and a woman only, thereby ruling gay couples out of federal benefits.
Yesterday, the court looked into California's Proposition 8, which bans gay marriage. A decision in both cases is expected in June, according to reports.
The 2nd Circuit Court of Appeals in New York ruled last year that law violated the equal protection clause of the US Constitution.
However, some campaigners are pessimistic of the chances of overturning Doma, with reports suggesting that the court may simply decide not to rule in the case – effectively leaving same-sex marriage legal in California but without the change that many are hoping for.
Nine US states – and Washington DC – currently permit same-sex marriage, while a further twelve states allow civil unions which provide variable amounts of state benefits.