The court is expected to hear two cases involving the issue of gay marriage over the next two days. Legal observers say their rulings will likely have a big impact on the future of same-sex marriage in America.
The court is considering overturning the Defense of Marriage Act, also known as DOMA, along with the constitutionality of California's Proposition 8, which bans same sex marriage.
A large march was held in downtown Chicago Monday night to support same-sex marriage. Similar demonstrations were held in 170 other cities.
It will be a significant decision for Illinois same-sex couples like Vernita Gray and her partner Pat Ewert. They have been together for three years and have a civil union that is recognized in Illinois. But when it comes to sharing federal rights and benefits like social security, they need the federal government to allow same-sex marriage.
"If something happens to me then my social security benefits will pass on to her and that part of my life I will be able to give to her," said Gray.
"What the court does with DOMA will determine how Illinois same sex couples are treated at the federal level," said Katherine Baker, Chicago Kent Law professor.
Some Chicagoans showed their support for same-sex marriage by rallying on Monday.
"We believe the court will strike down DOMA. It's the last vestige of another time. It doesn't have a place in our society today," said Ed Yohnka, American Civil Liberties Union of Illinois.
Meanwhile, Illinois lawmakers are considering a bill that would legalize gay marriage in the state- even as the Supreme Court makes a decision that could make their efforts moot.
Gay marriage opponents are rallying as well. Peter Breen from the Thomas More Society is helping to organize a huge demonstration and march Tuesday in front of the Supreme Court.
"We believe the people should decide and not the courts," said Breen.