Voters in Maine, Maryland and Washington approved gay marriage earlier this month.
But 31 states have amended their constitutions to prohibit same-sex marriage. North Carolina was the most recent example in May. In Minnesota earlier this month, voters defeated a proposal to enshrine a ban on gay marriage in that state's constitution.
The biggest issue the court could decide to confront comes in the dispute over California's Proposition 8, the constitutional ban on gay marriage that voters adopted in 2008 after the state Supreme Court ruled that gay Californians could marry.
The case could allow the justices to decide whether the U.S. Constitution's guarantee of equal protection means that the right to marriage cannot be limited to heterosexuals.
A decision in favor of gay marriage could set a national rule and overturn every state constitutional provision and law banning same-sex marriages. A ruling that upholds California's ban would be a setback for gay marriage proponents in the nation's largest state, although it would leave open the state-by-state effort to allow gays and lesbians to marry.