Israel has announced plans  to build a wall along its border with Egypt to keep illegal immigrants out and protect against terrorism.
Prime Minister Binyamin Netanyahu said yesterday that a barrier blocking the main infiltration routes along the 266-kilometre (166-mile) frontier will be constructed and advanced surveillance equipment installed. The total cost will be roughly £170m (one billion shekels).
Thousands of migrants from Eritrea, Ethiopia and Somalia have crossed the border over the past few years. Netanyahu said that, while refugees could still seek asylum in Israel:
This is a strategic decision to secure Israel's Jewish and democratic character.
So, the wall is largely a pragmatic measure, but physical barriers are fraught with the symbolism of oppression -- Gaza, the Berlin Wall. Just how common are these physical boundaries, why are they there, and which countries make most use of them?
Here are just a few examples of the separation barriers that exist along country borders. They offer a snapshot of the political tension around them and the complex technology they entail. Interestingly, walls have also been proposed on the borders of Pakistan/Afghanistan  and Russia/Chechnya. Is this the answer to cross-border conflict and problems such as smuggling and immigration? Please do leave comments below.
This new barrier will cover nearly all of the country's borders. Ehud Barak, Israel's defence minister, sums up the Israeli position:
We need a fence, as I said ten years ago, with all of our neighbours. With the Palestinians, we need two states for two people, a fence that will surround a solid Jewish majority. We will be here and they will be there.
Indeed, a barrier runs along the entire land border of the Gaza Strip. The section separating it from Israel was constructed in 1994, and consists of wire fencing with sensors. It is separated from Egypt by a wall of concrete and steel, built after 2004.
A similar barrier around the West Bank is under construction, but has attracted huge controversy. The International Court of Justice declared in 2004 that the erection of the barrier is "contrary to international law". Debate rages, as the wall (in some places, eight metres high) is not being built along 1949 Armistice lines, but within the West Bank, annexing areas  with substantial Israeli settlements, as well as water sources.
Settlers, and others , have also protested, arguing that none of the land should be relinquished. Construction paused in 2007, ostensibly due to lack of funds.
Elsewhere, Israel's borders are a physical legacy of past wars with neighbouring states. Its borders with Lebanon and Syria are covered by sophisticated security barriers with electronic surveillance and warning systems, a result of the 1949 Armistice and 1967 war, respectively.
Jordan -- the most peaceful of the borders -- is largely unbolstered, except for the section adjacent to the West Bank.
India -- the seventh-largest country in the world -- has also been constructing walls along its extensive borders since the mid-1990s.
Construction of a Kashmir barrier was completed in 2004, covering 550 kilometres (340 miles) of the disputed 740-kilometre (460-mile) ceasefire line; the aim is to prevent arms smuggling and keep Pakistani separatist militants out. The electrified barrier is between eight and 12 feet high, and also carries a network of thermal imaging devices and alarms, where power supply allows. It is well within Indian-controlled territory, though Pakistan claims that it violates bilateral accords.
Roughly half the tumultuous 2,900-kilometre (1,800-mile) border with Pakistan is similarly covered by barriers, and India plans to extend this  the whole length. A barrier on the border with Bangladesh is under construction to prevent illegal immigrants from entering. And it is hoped that another structure on the Burmese border will stem smuggling and terrorism.
About 554.1 kilometres (344.3 miles) of the 3,141-kilometre (1,951-mile) Mexico/US border is covered by a separation barrier, aimed at keeping illegal immigrants out and stemming the drugs trade. The barrier runs mainly along the border with New Mexico, Arizona and California, with construction ongoing in Texas, and consists of a series of short walls with "virtual fences" in between, including a system of sensors and cameras.
In the past 13 years, there have been approximately 5,000 migrant deaths along this border, according to the Human Rights National Commission of Mexico, a finding that was endorsed by the American Civil Liberties Union.