When the Berlin Wall was put up in Germany in 1961, it meant lots of people were cut off from their friends and relatives for many years.
Many people died trying to get across it, others managed to escape by tunnelling under it.
Sunday will mark 25 years since the wall was torn down.
But why was it built in the first place?
Jenny's been to Berlin to find out.
There's more on this in our special Berlin Wall guide.