IRA Arms: Dissidents thought to have some access
While mainstream republicans once considered the splinter groups to be amateurish malcontents, security chiefs in Northern Ireland and the UK say that they pose a clear and growing terrorist threat.
The leading dissident group is the so-called "Real" IRA.
It was born out of a republican split in October 1997 when the IRA's then quartermaster-general resigned over the direction Sinn Fein was taking in the peace process.
It was responsible for the Omagh bomb of 1998 and quickly took over from the older Continuity IRA as the leading home for dissidents.
It declared a ceasefire after the outcry over Omagh but security forces say that is has continued to recruit and has been behind recent incidents in Northern Ireland.
Its followers, thought to number at least 100, are thought to include IRA members who have been frustrated by the standing-down of operations.
Before it began attacks in London, it was thought to have access, at the very least, to IRA arms dumps containing explosives and detonators.
Some members are thought to be leading IRA bomb-makers.
RUC attacked
Its most recent activity in Northern Ireland shows a shift away from town centre explosions and a greater concentration on attacks on the security forces, including mortar attacks against an RUC station in Armagh.
A month later a device was left on a railway line near Ealing Broadway in west London.
Everything the dissidents have done has been intended to destabilise the peace process within Northern Ireland itself.
The attacks in London appear to indicate that the Real IRA is seeking to score propaganda and publicity coups by bringing large parts of London to a halt.
The units, comprising of just a handful of members living apparently ordinary lives, were designed to allow the IRA to leave operatives in London and elsewhere for long periods of time without exposing themselves to capture.
In security terms, operations planned and executed by an active service unit were generally far harder to stop than bombing operations planned and run from across the Irish Sea.
Arms access
The Provisional IRA has always been a skilled procurer of arms and the Real IRA appears be no less proficient.
It appears to be turning to Eastern Europe for weapons.
In July this year security services apprehended a major arms shipment in Croatia which they believed was heading for Northern Ireland.
The find was reported to include heavy machine guns, rocket-propelled grenades, plastic explosives and anti-tank weapons.
Weapons similar to those found in Croatia were intercepted in County Meath in the Irish Republic in October 1999.