Tuesday, millions of Americans will be able to sign up for health insurance through online exchanges. But there is growing concern some of these sites won’t be ready, and there may be technical glitches.

An exchange is like Expedia or Amazon for health insurance. That’s how Jon Gruber describes it, anyway. He’s an economist at MIT who helped write the Affordable Care Act.

“It’s a way to basically take this sort of complicated decision, put all your choices in front of you in a very clear way, and let you decide what kind of health insurance you want,” Gruber says.

But according to Michael Sparer, who teaches health policy and management at Columbia University’s Mailman School of Public Health, setting up a site that can do that isn’t easy.

“This is a huge undertaking,” he says.

There will be dozens of exchanges. Some states have their own. Others will use an exchange run by the federal government. Consultants have built these from the ground up.

Exchanges have to process a lot of data. A site has to figure out premiums, subsidies, and Medicaid eligibility, among other things.

“Most people are going to wait,” Sparer says. “They’re going to see what happens. They’re going to play this by ear.”

The exchanges may open Tuesday, but Americans have until mid-December to shop around for coverage that will begin next year.