Here are some of the issues behind the challenge to the Affordable Care Act:

What’s the lawsuit all about?

The ACA created health insurance marketplaces where people can shop for individual and family coverage. The law directs the federal government to set up exchanges in states that didn’t build their own, which was the case for about two-thirds of the country. People who sign up for insurance through the exchanges can get help paying for coverage through subsidies.

The challengers say that the law only authorizes subsidies through exchanges established by the state. They argue that the law was intentionally designed this way to pressure states to take on the effort of setting up their own insurance marketplaces. They contend the IRS illegally issued a rule in 2011 providing subsidies through the federal exchanges when it became clear that most states wouldn’t set up their own exchanges by 2014, when the marketplaces opened.

Who’s challenging the government?

The plaintiffs are four Virginians who do not want to buy health insurance, as mandated by the ACA. They argue the IRS illegally interpreted the ACA to authorize subsidies through federal-run exchange, and that without those subsidies, they would be exempt from the requirement to purchase insurance because they don’t earn enough. The challenge is funded by the Competitive Enterprise Institute, a libertarian think tank opposed to the ACA .

 Why do people think this case is so important to the future of ACA?

The subsidies are a critical part of the law, working with two other major ACA pieces – the guaranteed availability of health insurance and the individual mandate. The law prevents health insurers from denying coverage or charging people more because of their medical condition. To help offset the costs of sicker customers, the law requires most people to have insurance or pay an annual penalty for not having coverage. To make that coverage affordable, the law provides subsidies to low- and middle-income families, paid directly to insurers.

A ruling overturning the subsidies would cause more than 8 million people across the country to lose health insurance as a result of a ruling against the government, according to recent estimates. And it could wreak havoc on the insurance market.

Would there be a fix?

The immediate question is whether subsidies could be restored to those who’d suddenly lose them if the Supreme Court rules against the government. The easiest fix would be for Congress to pass a law that says federal exchanges can provide subsidies – but Republicans opposing the ACA would never go for that.

How will the court rule this time?

That’s hard to predict. Four justices from the court’s conservative wing were ready to throw out the entire law three years ago, but Chief Justice John Roberts found a way to uphold the individual mandate.

What’s next?

Oral arguments are scheduled for Wednesday but the court isn’t expected to release its decision until late June.