Good Friday Agreement: What is it?
1998 Good Friday Agreement has been in the spotlight because of the UK's departure from the European Union (EU).
But what is it and how did it come about?
BBC News NI looks at the historic peace deal and the current political fall out.
Good Friday Agreement, also known as the Belfast Agreement, was a political deal designed to bring an end to 30 years of violent conflict in Northern Ireland, known as the Troubles.
It was signed on 10 April 1998 and approved by public votes in Northern Ireland and the Republic of Ireland.
Northern Ireland was created in 1921 and remained part of the UK when the rest of Ireland became an independent state.
reated a split in the population between unionists, who wish to see Northern Ireland stay within the UK, and nationalists, who want it to become part of the Republic of Ireland.
From the late 1960s, armed groups from both sides, such as the Irish Republican Army (IRA) and Ulster Volunteer Force (UVF), carried out bombings and shootings - and British troops were sent to Northern Ireland.
roubles lasted almost 30 years and cost the lives of more than 3,500 people.
Good Friday Agreement is based on the idea of co-operation between communities.
It set up a new government for Northern Ireland, representing both nationalists and unionists. 
BBC News NI asks young people what their understanding of the Good Friday Agreement is
Westminster government gave this government control over key areas such as health and education - a process known as devolution.
A new parliament, the Northern Ireland Assembly, was set up and sits in Stormont, in Belfast.
Other parts of the agreement are about respect for people's rights, whichever part of the community they come from. 
It says:
As part of the agreement:
After Brexit, Northern Ireland became the only part of the UK to have a land border with an EU country - the Republic of Ireland.
Checks are required on goods transported between the UK and the EU's markets.
But both sides agreed this should not happen on the Irish border, to protect the Good Friday Agreement, because it was feared the cross-border co-operation could be threatened if new checkpoints were set up.
During the Troubles, people crossing the border were subject to British Army security checks - and surveillance watchtowers were placed on hilltops. 
And although the agreement does not specifically refer to the border, it does mention removing all security installations.
keep the border clear, the UK and EU agreed the Northern Ireland Protocol. Goods are checked to ensure they comply with EU rules when they arrive in Northern Ireland from the rest of the UK (England, Scotland and Wales).
Supporters of this plan, including the nationalist Sinn Féin party, say it is necessary to protect the Good Friday Agreement.
But unionist parties, including the Democratic Unionist Party (DUP), say this actually undermines the agreement because it separates them from the rest of the UK.
DUP has refused to take part in power-sharing until its concerns are addressed. 
means there has been no devolved government - set up by the Good Friday Agreement - in Northern Ireland since February.
UK government has also criticised the protocol, saying it has been "upsetting the balance of the Good Friday Agreement".
It is trying to ditch parts of the agreement, with new legislation.
urrent stand-off with the EU, which says the protocol is the "one and only solution" to protecting the peace process in Northern Ireland.
Negotiations between the UK government and the EU to resolve differences over the protocol are continuing.