The Ulster Unionist Party dominated Northern Ireland's politics from partition in 1921 until its collapse at the 2005 British general election, when it lost all but one seat.
It is now the second largest unionist party, after the Democratic Unionists.
The UUP was founded in 1905 to resist Home Rule for Ireland. At the introduction of partition, the party enjoyed an in-built electoral majority that effectively shut out its rivals and created a one-party state.
This situation allowed successive UUP prime ministers to ignore the inequalities and discrimination suffered by the province's Catholic majority. Attempts at reform by Prime Minister Terence O'Neill in the 1960s failed, and it was during his tenure that the Troubles flared up for the first time.
Brian Faulkner served as the last Ulster Unionist prime minister before direct rule was introduced in 1972.
The 12th party leader, David Trimble, was instrumental in bringing about the Good Friday Agreement, and his contribution was recognised in the Nobel Peace Prize he shared with David Hume.
In his 1998 Nobel acceptance speech, Trimble alluded to the role his party had played in fomenting many of Northern Ireland's problems by acknowledging that unionists had built a 'cold house' for Catholics.
Trimble served as first minister on the Northern Ireland executive until direct rule was reintroduced on 14 October 2002 amid allegations of republican spying and the failure of the IRA to demonstrate that it had left violence behind for good.
At the 2005 general election, Trimble was defeated and his party - its credibility badly damaged by perceived concessions to republicans - was almost wiped out at the polls.