The modern party was founded in 1970 when Provisional Sinn Fein split from Official Sinn Fein, although it derives its name from an organisation founded by Irish nationalist Arthur Griffith in 1905.
It is still regarded by many commentators as the political wing of the Provisional IRA (which split from the Official IRA at the same time), although it denies that any link still exists.
Sinn Fein is a hardline nationalist party dedicated to the creation of a united Ireland. It advocates strong cross-border bodies as a step towards achieving that goal and the maintenance of the Irish Republic's territorial claim to Northern Ireland.
It is a strong supporter of the Good Friday Agreement, but accuses unionists of undermining the deal over the issue of IRA arms decommissioning.
Sinn Fein had been barred from taking up its executive seats by the assembly's first minister, David Trimble.
The Northern Ireland Assembly was suspended and direct rule 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.
Since the early 1980s, Sinn Fein has slowly gained strength and political power, and has now overtaken the more moderate SDLP as the largest nationalist party.
But its two MPs, party leader Gerry Adams and Martin McGuinness, continue to abstain from taking their seats at Westminster because they refuse to take the required oath of allegiance to the queen.
It is the only party to hold seats at Westminster and in the Republic of Ireland's Dail.