The Good Friday Agreement is being commemorated in a series of events in April to mark its 25th anniversary.
Signed on April 10 1998, it largely brought to an end the three decades of violence in Northern Ireland known as the Troubles.
US president Joe Biden and former president Bill Clinton are both set to visit Northern Ireland as part of events to commemorate the historic peace deal.
Read more: Stormont instability a lingering regret from Good Friday Agreement, says Bertie Ahern
The agreement has played a significant role over the past quarter-century in shaping Northern Ireland, from its power-sharing political institutions to British-Irish relations and the handling of Brexit.
Here is a summary of the key points of the agreement:
What is the Good Friday Agreement?
The Good Friday Agreement, also known as the Belfast Agreement, was a political deal aimed at bringing to an end the three decades of violent conflict in Northern Ireland known as the Troubles.
It was signed on April 10 1998 and was later approved by referendums held both in Northern Ireland and the Republic of Ireland.
The agreement led to a new system of devolved government to Northern Ireland on the basis of power-sharing between unionists and nationalists through the Stormont Assembly and Executive.
It also created new institutions between Northern Ireland and the Irish Republic, and between the Republic and the UK.
The deal followed intensive talks between most of Northern Ireland's political parties and the British and Irish governments, with a US special envoy chairing discussions.
What does the agreement say?
The Good Friday Agreement includes provisions for the system of government of Northern Ireland within the United Kingdom, known as "Strand One" of the agreement.
It also deals with the relationship between Northern Ireland and the Republic of Ireland (Strand Two) and the relationship between the Irish Republic and the UK (Strand Three).
The agreement says Northern Ireland's place within the UK can only change if a majority in the region support this through a referendum, also known as a border poll.
It recognised that people born in Northern Ireland can identify as Irish, British or both, and hold both Irish and British citizenship if they choose, regardless of any change in the status of Northern Ireland.
Paramilitary decommissioning and prisoner releases
The agreement also sought to tackle the challenging issues in a bid to normalise Northern Ireland society after decades of violent conflict during the Troubles.
The agreement committed participants to "exclusively democratic and peaceful means" for resolving their differences.
Parties pledged to "use any influence they may have" to bring about the decommissioning of all weapons held by paramilitary groups.
Both governments committed to the early release of many prisoners involved in violence during the Troubles. More than 400 paramilitary prisoners were released in the two years after the agreement was signed.
The UK government also pledged to reduce the presence of the British military in Northern Ireland.
To examine policing arrangements, a commission was set up which led to the establishment of the Police Service of Northern Ireland (PSNI) to succeed the Royal Ulster Constabulary (RUC).
What were the referendum results?
The Good Friday Agreement was backed by voters in referendums held in both Northern Ireland and the Irish Republic on May 22 1998.
Voters in Northern Ireland were asked to approve the multi-party agreement section of the deal, while voters in the Republic were asked to approve both the multi-party agreement and certain changes to the Irish constitution.
In Northern Ireland 71% of voters backed the deal, while in the Republic the figure was 94%.
Did the agreement hold?
Support for the agreement led to the establishment of a new devolved government in Northern Ireland.
However, the Stormont institutions were soon suspended due to a lack of progress in paramilitary decommissioning, which damaged unionist confidence in the agreement.
Power-sharing was again restored in 2007 but the Executive and Assembly have collapsed several times since then and they are currently not operating.
Since last year the DUP as the largest unionist party has been blocking Stormont power-sharing in protest against Brexit's Northern Ireland Protocol.
How has Brexit affected the Good Friday Agreement?
While the UK leaving the European Union does not directly impact the Good Friday Agreement, it has made British-Irish relations more complicated and strained.
The text of the 1998 peace deal had assumed both the UK and Republic of Ireland would be in the EU.
But Brexit meant Northern Ireland became the only part of the UK to have a land border with the EU, as the Republic is still a member state of the bloc.
During protracted Brexit negotiations, all sides have argued they have sought to protect the Good Friday Agreement - despite their positions often running contrary to each other.
The Northern Ireland Protocol agreed between the UK and EU sought to maintain an open land border for post-Brexit trade with the Republic, with checks instead at Northern Ireland's ports.
The Windsor Framework deal agreed earlier this year has sought to reduce new trade barriers between Great Britain and Northern Ireland following unionist anger over the arrangements.
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