Russian President Vladimir Putin has signed a treaty to make Crimea a part of Russia. It's after the Crimean parliament formally applied to join its neighbouring country.
Crimea has been part of Ukraine since 1954.
On Sunday more than 96 percent of people who took part in a vote in Crimea said they wanted it to join Russia.
President Vladimir Putin says the vote was fair.
But leaders in Europe and the United States say the vote was illegal.
It comes after months of unrest in Ukraine.
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Many people living in Crimea refused to take part in the vote.
A vote like this is called a referendum and usually they are announced a long time before voting actually takes place.
In Crimea the vote was announced and arranged in under two weeks.
When the vote was announced some Ukrainian TV channels were made unavailable in Crimea. Some were replaced with Russian channels.