The UUP has long been the largest party in Northern Ireland.
But the peace process and the difficulties that have come with it has seen the party's membership divide and many of its supporters switch to the hardline Democratic Unionists.
At the 1997 general election, 10 months before the signing of the Good Friday Agreement, a third of Northern Ireland's voters supported the party, delivering it 10 of the 18 parliamentary seats.
The following year, the UUP took 28 of the 108 seats in the Northern Ireland Assembly, making party leader David Trimble the First Minister-designate.
But as Mr Trimble's leadership and peace process strategy came under fire from many among his own party, that support slipped - devastatingly so at the 2001 general election.
Rather than emerging from the election as the unassailable leader of the unionist community, Mr Trimble witnessed his party finish with just six seats - three of the losses at the hands of Ian Paisley's Democratic Unionists.
Party of partition
The UUP, formerly known as the Official Unionist Party, was the absolute political master of Northern Ireland from partition in 1921 until the imposition of direct rule in 1972.
The central plank of UUP policy remained maintaining the link between Northern Ireland and the rest of the United Kingdom.
But the actual nature of that link - and what relationship with the Republic of Ireland - has been the defining characteristics of the party's political history.
When the civil rights movement emerged in the 1960s and demanded political and social change of the unionist government, the party faced the first of many policy splits.
The first reform-minded leader of the party during that decade, Terrence O'Neill, sparked fury among unionists after he invited the Irish Taoiseach to Belfast for talks and advocated social and political change to what had long been considered a "Protestant state for a Protestant people".
The last prime minister of Northern Ireland in 1972, Brian (later Lord) Faulkner, initially resisted any form of powerharing arrangements and sparked nationalist fury by introducing internment without trial.
But the introduction of direct rule came as a massive body blow to the party. The closure of Stormont brought to an end its half-century of control of events in Northern Ireland and eventually led to a realignment within the party in which the working class members gained more control.
Faulkner eventually agreed to powersharing and a cross-border body as part of the 1973 Sunningdale agreement - but the party divided as many members sided with the Democratic Unionists and various loyalist groups to bring down the deal and the leader.
More than a decade later, the UUP was utterly opposed the Anglo-Irish Agreement which introduced a role for Dublin in Northern Ireland affairs through a joint ministerial council; its opposition led to its closest ever co-operation with the Democratic Unionists.
During the 16 years leadership of James (now Lord) Molyneaux (1979 - 1995), the party pursued a number of devolution strategies which fell short of powersharing. On powersharing itself, Molyneaux remained clear: Northern Ireland's divisions could not be healed through a "shotgun marriage between those who are British and those ... atttracted to the idea of Irishness." It was a view apparently held by a majority of the party.
David Trimble's taking of the helm in 1995 marked a new direction. He took the party into the political talks which eventually led to the Good Friday Agreement.
Mr Trimble's role in negotiating the Good Friday Agreement led to him jointly winning the 1998 Nobel Peace Prize with the SDLP leader John Hume - an award that some observers suggested had possibly been made a few years too early.
Mr Trimble historically secured his party's backing to work in the powersharing assembly and cross-border political bodies, but his leadership quickly became dogged by the vexed question of paramilitary arms decommissioning.
After one false start, the Northern Ireland executive was established when the Ulster Unionist council backed David Trimble's stance on 27 November 1999. The decision - by 480 votes to 349 - paved the way for a power-sharing executive, linked to decommissioning and marked a sea-change in Ulster Unionist thinking.
When the executive was suspended within weeks amid Mr Trimble threat to resign over a lack of movement on decommissioning, the party's nationalist critics said that it had failed to learn the lessons of the past three decades.
But Mr Trimble secured his party's support on a second occasion after the a comprehensive deal in May 2000 which sought to address the concerns of all participants in the political process.
The party remains ruled by the 800-strong Ulster Unionist Council, a body that has come under the spotlight since 1998 because of its pivotal role at critical stages of the peace process. The most controversial aspect of the council is that the Orange Order is allowed to send voting representatives to its meetings - even though they may be more closely aligned with other shades of unionism.