The Berlin Wall split Europe in two and divided the German city of Berlin for almost 30 years.
It was constructed over night, much to the surprise of people either side of it, and stopped people moving from one part of the city to the other.
It was eventually knocked down in 1989 and to mark the 25th anniversary of that historic event here's Newsround's guide to what it was, how it was built and the effect it had on people's lives.
Why was the Berlin Wall built?
At the end of World War Two Germany surrendered to the Allies, a group of Western countries including Britain, America, France and the Soviet Union (a collection of Eastern European countries run by Russia).
The Allies decided to divide control of Germany between themselves and so each took responsibility for a different part of the country. Britain, America and France took over the areas in the west of Germany and the Soviet Union controlled the east.
Berlin was in the Soviet zone but as the capital of Germany it was decided that it would also be divided into four areas, one controlled by each of the four countries.
Tension and division
It soon became clear that the Soviet Union had very different ideas to the others about how their section should be run.
By 1949 Germany had become two separate countries - The Federal Republic of Germany (West Germany), run by Britain, America and France, and the German Democratic Republic (East Germany), run by the Soviet Union.
West Germany was run in a similar way to how Britain and America is today with people free to move around, listen to whatever music they liked and express their opinions.
East Germany was much stricter with tight rules on how people should behave and a police force that monitored what they did.
As the years went on thousands of people a day were escaping from East Germany to the West and that was the main reason the Berlin Wall was built.
How was it built?
In 1961 the leader of the Soviet Union, Nikita Khrushchev, ordered that a wall be built between East and West Berlin to stop people leaving.
It was constructed really quickly overnight on the 13 August so many people woke up to find they were trapped on one side, often separated from their friends and family in the West.
The wall covered all the land in a line between East and West Berlin so there was nowhere for people to cross.
It was originally a 96 mile barbed wire fence but was later rebuilt because people were able to climb over it.
How big was it?
The finished wall was made up of a 66 mile concrete section that was 3.6 metres high, with a further 41 miles worth of wire fencing and more than 300 look out points along it, manned by guards to stop anyone crossing.
The wall became a symbol of the division in Europe between the west and the east and was called the 'iron curtain'.
Soviet Union leaders said it was a protective shell but Britain, American and France saw it as a prison which stopped people leaving the east.
What was life like?
Around 5,000 people tried to escape over the wall but it was very difficult and dangerous.
It is thought more than 100 people were killed trying to cross the Wall in the 29 years between 1961 and 1989.
Life for people in East Berlin was difficult. People who had previously worked in West Berlin lost their jobs.
They were also separated from friends and family.
What happened to the wall?
In the 1980s revolutions against the way the Soviet Union controlled lots of countries in Eastern Europe started to take place.
People in the east wanted more freedom to go where they wanted, listen to whatever music they liked and voice their opinions freely.
As part of these protests people in East Germany began demanding that they be allowed out. After hundreds of East Germans escaped via neighbouring countries like Hungary and Czechoslovakia the government in East Berlin found it increasingly hard to stop the calls for people to be allowed to cross into West Germany.
On 9 November the leader of East Germany gave a TV speech where he said the border between east and west would be opened immediately.
Thousands of people from East Germany went to the wall and demanded the guards open the gates. At about 10.45 at night they did just that and thousands of people crossed over to West Germany for the first time in their lives.
Hundreds of West German people were waiting for them and celebrated the historic moment, some even danced on top of the wall.
The date on which the Wall "fell" is considered to have been 9 November 1989, but the whole wall was not torn down immediately.
Over the following weeks, many people started to smash it down with sledge hammers and took pieces of the Wall to keep because it had become so symbolic.
The government finally destroyed the Wall in 1990 although parts of it have been left for people to see today.
Discussions between East and West Germany also started in 1990 about reuniting to become one country again and that is the Germany we know today.