Petrol bombs and bricks have been thrown at police during heavy riots on the streets of west Belfast.
A water cannon was used to try to stop the 200 or so rioters as trouble flared in Northern Ireland's capital.
The violence comes ahead of the biggest day in Northern Ireland's marching season, in which thousands of people take part in traditional parades.
Lots of people still disagree over who should rule Northern Ireland.
Nationalists, who were rioting, want it to join the Republic of Ireland and form a united Ireland, while unionists who take part in the marches think it should be part of the UK.
Unionists, who are also known as loyalists, are mainly protestant. Nationalists, who are also known as republicans, are mainly Catholics.
In the past, the differing views of the two groups have led to a lot of unrest.