"We're going to build a wall" - it's something we have heard US President Donald Trump say many times since he launched his presidential campaign.
The president wants a physical barrier along the border between Mexico and the US to prevent people from crossing over it illegally.
The border wall has proved to be a controversial topic, with many people disagreeing about whether or not it's a good idea.
Some of the existing border barriers have been replaced since Mr Trump entered the White House, but by March 2019, work to extend what is there had only just begun.
Newsround has been to the US-Mexico border to find out more about Mr Trump's proposed wall and how it affects people's lives.
Read on to find out more about US-Mexico border wall.
Building a wall was one of Donald Trump's key promises when he was campaigning to become US president.
It was an idea that divided the country, but it won Donald Trump many supporters.
It is over two years into his presidency, but he still wants to deliver on that promise.
A wall at the border between the US and Mexico would be designed to prevent people from entering the US illegally.
I would build a great wall, and nobody builds walls better than me, believe me, and I'll build them very inexpensively. I will build a great great wall on our southern border and I'll have Mexico pay for that wall.
Currently, there is a total of 650 miles of barrier along the border, which is 1,900 miles long.
The US Customs and Border Protection agency has said that, so far, it has replaced about 14 miles of scrap metal barrier with a bollard-style wall in San Diego, built two miles of wall in California, and 20 miles of new wall in Santa Teresa in New Mexico.
However, Donald Trump believes the existing barriers are not enough to stop people attempting to cross and he wants to build more.
In a televised speech, he stated: "All Americans are hurt by uncontrolled, illegal migration.
"We are out of space to hold them and we have no way to promptly return them back home to their country," he continued.
President Trump wants to build a 1,000-mile wall made out of steel to prevent people from crossing the US-Mexico border.
He believes the rest of the terrain along the border - including mountains and forests - should be enough to prevent people from attempting to cross.
But building a wall of this size is going to cost a lot of money.
We are not paying for that stupid wall.
Originally he suggested that Mexico should pay for the wall, but it became clear early on that the Mexican government would not pay.
Now, President Trump is appealing to his own government - and therefore, to US taxpayers - to pay for the wall.
He estimates that he would need $5.6 billion (the equivalent to £4.4 million) to build a steel barrier across the border.
This has not been well received by his opposing party - the Democrats.
The issue of how to pay for the wall caused a stand-off within Congress - the body which creates, debates and makes laws in the US.
The Democrats were very successful in the US mid-term elections, which gave them more power to be able to refuse money for the project.
With no decision being made earlier this year, Congress was unable to pass a spending bill - and the government was forced to partially close.
In March 2019, the first funds for the wall were given the go-ahead. A total of $1 billion (about £758 million) was authorised to be sent to army engineers.
Some have suggested that a barrier is immoral. Then why do wealthy politicians build walls, gates and fences around their homes?
Polling has showed that most US citizens don't agree with building the wall.
A mid-November CBS poll found that 59% disagreed with it.
However, those that agree with the wall being built are working hard to make their opinions heard.
Immigration - particularly illegal immigration - is a very important issue to many Americans.
Those who agree with the wall being built believe that the US is overwhelmed by illegal immigration.
Their opinion is that people who come to live and work in the country, without official permission from the government, are taking advantage of the country's privileges and freedoms.
Many feel the wall will prevent illegal immigrants from coming into the country as easily.
...too many illegals are taking advantage of the United States taxpayers with no means of ever contributing to our society.
A GoFundMe page was set up by US Army veteran Brian Kolfage. He believes that in the event that Congress refuses to pay for the wall, Donald Trump's supporters could pay for it instead.
"If the 63 million people who voted for Trump each pledge $80, we can build the wall," he stated on his fundraising page.
He has already raised $22.2 million. It's a long way off the $5.6 billion needed, but it is still a very large amount to raise.
But what about the majority of citizens who do not believe the wall is a good idea?
The symbol of America should be the Statue of Liberty, not a 30-foot wall.
The majority of US citizens believe that the wall is a bad idea or that it shouldn't be a priority for the president.
"President Trump has appealed to fear, not facts. Division, not unity," stated Democrat Senator, Chuck Schumer, in reaction to Trump's televised address.
There is some evidence that the statements Trump made during his address were not factually correct.
For example, President Trump claimed that there is a crisis in the number of people attempting crossing the border illegally in the past few years.
But statistics have showed that the number of arrests at the US-Mexico border have decreased a lot since the early 2000s.
Currently, there is an average of 400,000 arrests at the border each year, compared to more than a million arrests in the early 2000s.
Aside from the questioning of the facts, many Democrats feel that building a wall would send the wrong message about US immigration.
Senator Schumer referenced the Statue of Liberty as a better symbol for the country.
The Statue of Liberty is also known as the Immigrant's Statue, as it was seen as a representation of hope and welcome to the millions of people who immigrated to the US through New York more than a century ago.
But this does not mean that the Democrats are completely against more security at the border between the US and Mexico.
They have stated they would support increased security, but not in the form of a wall and not in the form of a budget of $5.6 billion.