During his presidential campaign, Donald Trump passionately promoted construction of a border wall between the U.S. and Mexico and insisted that Mexico would pay it. One year into office, he remains determined to get something built, even if it’s not as vast as he used to suggest. But…

1. What’s Trump asking for?

He’s seeking $18 billion to $20 billion over 10 years for border security, as part of a fiscal 2019 spending package, and part of that would pay for construction of new and replacement barriers. On Jan. 10, he offered to support a Democratic goal — providing legal status to some 800,000 young, undocumented immigrants under the Deferred Action for Childhood Arrivals program — in exchange for money for the wall as part of down-to-the-wire negotiations for a short-term budget. Then, two days later, he rejected a bipartisan plan for that very deal. This created an impasse, increasing the odds that the government would run out of money and shut down.

2. What might the wall look like?

Though Trump denies changing his position, he no longer seeks a monolithic, 30-foot-tall concrete wall stretching for more than 2,000 miles (3,218 kilometers). Plans now call for a more modest, 722-mile barrier that is a mix of wall and fencing, mostly updating what’s been in place for decades, while relying on drones and other methods to secure the rest.

3. What’s been done so far?

Eight wall prototypes are now being tested in a desert outside San Diego, California. Four are primarily concrete structures, with two made mostly of metal and two others being hybrid designs of concrete, metal bars and steel plating. The mock-ups are being tested for their ability to repel attempts to climb over, smash through or tunnel underneath. U.S. Customs and Border Protection will select the best designs and begin the process of awarding contracts for actual construction once there’s money.

4. How much would a wall cost?

Estimates range from as low as $8 billion to as much as $67 billion or more, depending on whom you ask and the number of miles of wall that get built. Based on Trump’s 2017 budget request for $2.6 billion to plan, design and build 75 miles of wall, Democratic Senator Claire McCaskill’s office estimated the per-mile cost would be about $37 million, or nearly $67 billion for the entire 2,000-mile border. Congressional Republicans have said they expect a wall to cost from $12 billion to $15 billion, based on the cost to rebuild existing border fencing covering a third of that distance. These projections, however, don’t include the cost of land acquisition.

5. Doesn’t the government own the border?

Far from it. Two-thirds of the land is private or state-owned, much of it in Texas. The Trump administration could seek to use eminent domain to seize land needed for a border barrier, as well as support roads and other infrastructure, though it would likely face costly legal challenges that could delay construction for years.

6. How could Trump get Mexico to pay for the wall?

Mexican President Enrique Pena Nieto has emphatically refused to fund the border wall. Trump has suggested that the U.S. can recoup wall expenses from Mexico via alternative methods, including by cutting its trade surplus with the U.S. He’s also floated the option of invoking the Patriot Act to cut off or tax remittance payments to Mexico from Mexican immigrants living in the U.S. Mexicans sent home $25.7 billion in remittances in 2016, according to the Banco de Mexico.

7. So who will pay for it?

If Mexico can’t be made to pay, U.S. taxpayers, most likely.

8. How much wall exists already?

Barriers span 653 miles of the U.S.-Mexico border, mostly along the western half. Much of the southern borders of California, Arizona and New Mexico have existing barriers, ranging from 18-foot-tall iron fencing and corrugated metal to makeshift vehicle barriers and barbed wire.

9. Would a wall even work?

Most experts doubt that a physical wall would do much to reduce the $150 billion in illegal drugs that pour into the country each year. As part of a set of tools to combat illegal immigration, however, physical barriers could help. Israeli Prime Minister Benjamin Netanyahu praised Trump’s plan last year, saying a fence built along Israel’s border with Egypt has been a “great success” in keeping out migrants, mainly from African nations.