"Donald Trump’s promise to build a "big, beautiful wall" between the US and Mexico was a rallying cry throughout his election campaign.
As president, Mr Trump has asked for design ideas for the barrier, with a chosen few to be selected this month for a prototype showcase this summer in San Diego, California.
Mr Trump says he wants a wall along half the 2,000-mile (3,100km) border - with nature, such as mountains and rivers, helping to take care of the rest.
However, the route crosses difficult terrain, is home to a lot of wildlife and cuts across land owned by Native American tribes as well as private citizens.
So is building the wall actually possible? Here are some of the obstacles Mr Trump will have to overcome. Read the BBC News article, which is filled with graphics: