Election profile: Democratic Unionist Party
For more than three decades, the Democratic Unionist Party was the natural party of protest for unionists in Northern Ireland, the outsiders who challenged the establishment Official, or Ulster, Unionists.
But since 2007, when Ian Paisley agreed to share power at Stormont with Sinn Féin, the DUP has become a party of government.
When Ian Paisley became MP in 1970, he stood as a Protestant Unionist, but his party was relaunched as the Democratic Unionist Party in 1971.
The party drew support from Protestants and was fiercely opposed to any moves towards involving the Republic of Ireland in Northern Ireland affairs.
During the 1990s, the DUP pulled out of talks that led to the Good Friday Agreement in protest against the decision to invite the IRA's political wing, Sinn Féin, to enter negotiations.
The DUP benefited from growing unionist unease over power-sharing and the failure of the IRA to decommission, overtaking the Ulster Unionists at the Northern Ireland Assembly elections in 2003.
Nine DUP MPs were subsequently elected.
In 2006, the party was a key player in the St Andrews' Agreement in Scotland. A year later, the party made its historic decision to share power with Sinn Féin.
Ian Paisley served as first minister until 2008 when Peter Robinson took over.
But, in the most surprising result of the 2010 General Election, Mr Robinson lost his East Belfast Westminster seat to Alliance's Naomi Long.
Regaining East Belfast is a key DUP objective in 2015, and it is one of four seats covered by an electoral pact between the DUP and UUP.
The DUP currently has eight MPs and is the fourth largest party at Westminster - a fact it highlighted when it was excluded from UK pre-election television debates.
The relationship between Peter Robinson and Martin McGuinness has never appeared as friendly as that between the deputy first minister and Mr Paisley.
It continues to appear strained, notably over the issue of welfare reform.
The DUP, which holds the ministry in the Northern Ireland Executive responsible for the implementation of the bill, and Sinn Féin, whose leadership in the Republic of Ireland has championed an anti-austerity approach, seem to be at loggerheads.
It was thought a deal had been done during talks at Stormont in December 2014, but in March - on the day the final stage of the bill was due to pass through the assembly - Sinn Féin withdrew its support and the bill's passage was halted.