As President Donald Trump marks 100 days in office, he is vowing to keep his campaign promise to build a wall along the border with Mexico.
Despite widespread skepticism and Mexico's refusal to pay for the wall, as Trump has demanded, the US government has been soliciting bids and test sections could be built as soon as this summer.
Physical barriers are as old as humanity and have proven to be effective over the centuries at protecting borders. But fences can also have unwanted consequences, like destroying city neighborhoods, harming the environment and preventing innocent victims from reaching safety.
Here is a look at some of the world's barriers. Some are rather simple, while others are massive enterprises that cost billions. Generally they have been paid for through standard budgeting procedures, with none sparking a funding dispute of the kind that has emerged with Mexico.
The United States And Mexico
A third of the U.S.-Mexico border is already studded with an assortment of fences, but closing off the rest will be no easy task.
Much of the border in Texas is blocked by the Rio Grande or other natural barriers, or runs along land owned by private citizens, many of whom oppose the wall.
There also is much skepticism over whether the fence would stem drug trafficking, illegal immigration or gang violence.
Israel and the West Bank
Israel began construction of its 150-mile (250-kilometer) separation barrier in 2002 in response to Palestinian suicide bombings that killed more than 1,500 people.
Israel says the structure is a crucial defense measure. But because it frequently juts into the occupied West Bank, the Palestinians see it as a land grab that impedes their dream of establishing an independent state.
India needed just two years to build a 550-kilometer (340-mile) fence along the disputed border area with Pakistan a decade ago. Now a high-tech barrier, it is laced with thermal imaging devices, motion sensors and lighting systems along a mined strip of land between two rows of coiled razor wire.
The Indian military calls it an "anti-infiltration obstacle system," designed to keep Pakistan-based rebels from crossing over in their fight for Kashmir's independence or merger with Pakistan.
Hundreds of thousands of soldiers are deployed on both sides, sometimes standing eye to eye, making it one of the world's most militarized areas.
A UN-controlled buffer zone and frontier stretches 180 kilometers (120 miles) from coast to coast across the tiny island, separating the breakaway Turkish Cypriot north from the internationally recognized south. The division dates to 1974, when Turkey invaded following a coup by supporters of union with Greece.
The frontier is rarely marked by a physical wall, with the notable exception of the capital, Nicosia, where one runs through the heart of the Old City. At its narrowest point, just a few meters (yards) separate Greek Cypriot national guardsmen from Turkish and Turkish Cypriot troops.
Decrepit, crumbling buildings inhabiting this no-man's land stand in stark contrast to the trendy bars and coffee shops that have sprouted up nearby.
The Korean Peninsula
The Demilitarized Zone, a Cold War vestige, was created in 1953 after the Korean War ended with an armistice. Running from coast to coast, the 4-kilometer (2 1/2-mile)-wide, 248-kilometer (154-mile)-long DMZ bisects the peninsula and forms the de-facto border between the Koreas.
Mined on both sides, with a razor-wire fence, tank traps and hundreds of thousands of troops, the DMZ is the world's most heavily fortified border. More than a million mines are believed to be buried within it. Some 28,500 U.S. troops are stationed in South Korea as deterrence against potential aggression from North Korea.
The DMZ also includes the truce village of Panmunjom, which despite animosities is a popular tourist spot drawing visitors on both sides.
Kenya and Somalia
Kenya decided to erect a wall along its Somalia border after an April 2015 attack by Islamic extremists killed 148 people, most students, at a university.
Initially the Kenyan government announced a 700-kilometer (435-mile) wall, but officials say just 30 kilometers (18 miles) of fencing have been completed.
Some doubt whether the wall will be effective given the busy cross-border smuggling trade, which benefits powerful officials on both sides.
Hungary built fences along its southern borders with Serbia and Croatia in 2015, when thousands of migrants were passing through each day headed to Germany and other destinations in western Europe.
Prime Minister Viktor Orban is a staunch opponent of migration, and Hungary has been accepting only a few hundred asylum-seekers a year.
Human rights groups and UN agencies have been critical of the fences and the country's restrictive asylum policies. There are increasing reports from aid groups about police brutality against migrants being pushed back to Serbia, allegations Hungary denies.
A concrete wall, one kilometer (half a mile) long and four meters (13 feet) high, is the finishing touch on an already elaborate defense system against migrants in the northern French port city of Calais.
Britain doled out the 2.7 million euros ($2.9 million) to pay for the edifice along the highway leading to the Calais port, much used by truckers crossing the English Channel.
Migrants flocked to Calais for years, hoping to sneak into Britain, often in freight trucks. A huge makeshift camp was dismantled in October, with thousands of migrants bused to special centers.
Spain has built six-meter (20-foot) layered border fences around its two North African enclaves, Ceuta and Melilla, to dissuade migrants, now mostly from sub-Saharan countries, from entering them via Morocco.
The fences, along with cooperation between Spain and Morocco, have helped keep out migrants. But rights groups say both Spanish and Moroccan security forces have beaten migrants caught scaling the wall. They also criticize Spain's use of razor-blade coils — which have caused many injuries — and its expelling those who make it over without letting them apply for asylum.