Violence broke out for the second night in a row in Northern Ireland's capital city, Belfast.
15 police officers were injured, as petrol bombs, bricks, fireworks and stones were thrown at them.
Police were trying to to break up the fighting between two groups of people called loyalists and republicans.
These groups share a long history of violence in Northern Ireland that goes back hundreds of years.
Loyalists are mainly Protestants who want the country to stay part of Great Britain, whereas republicans are mainly Catholics who want the area to join the Republic of Ireland.
The most recent clashes began when loyalists became angry that republicans were allowed to parade through the streets without restriction.
Last night's violence follows trouble that erupted on Sunday night, when 47 police officers were injured during disturbances in the same area.
But there's a feeling that this violence has been building up for some time.
Police who work in Belfast say they feel unhappy that it is unsafe for them to protect the streets.
"The community of north Belfast needs to see a resolution to this issue now," said Assistant Chief Constable Will Kerr, a top policeman there.
"We cannot afford to wait and we cannot have night after night of violence on our streets," he said.