Although the Provisional IRA was always by far the biggest and most dangerous of the violent republican groups, several smaller separate groups exist, some of them breakaways from the main IRA, which is now defunct.
Although co-operation has been known, with individuals occasionally switching allegiance, the cells are independent and often localised entities.
The group which has claimed responsibility for killing a police officer in Craigavon on Monday night, the Continuity IRA (CIRA), has existed for two decades. A spokesman for its political wing, Republican Sinn Fein, rejected the description of Continuity IRA activity on police as murder attacks, saying: "There were simply a number of military engagements designed to kill them."
The CIRA originated from a split within IRA Fein in the mid-1980s, when some republicans broke away in protest against the developing peace process. They claimed the IRA and Sinn Fein would eventually "go political" and lose sight of traditional republican goals.
The Continuity IRA has been responsible for two internal killings, as well as attacks on police officers, vehicles and premises. It was described in a security assessment as "active, dangerous and committed, and capable of a greater level of violent and other crime".
Another group, the Real IRA, mounted the weekend attack that killed two soldiers in Antrim, and will always be associated with the 1998 bomb that killed 29 people in Omagh. Two years after Omagh, it staged a "spectacular" by launching a rocket attack on the London headquarters of MI6. This caused little damage, but the rocket hitting its target meant it was regarded as a significant coup for a small group. The organisation was responsible for two separate shooting incidents in 2007 in which police officers were wounded.
It was formed in 1997 after the resignation of five members of the mainstream IRA's executive who objected to Sinn Fein's sponsorship of the peace process. Its first leader was Micky McKevitt, now jailed, who, as IRA quartermaster, took some IRA weaponry with him when he quit.
Two new groupings have emerged, both styling themselves ONH, a reference to the Irish name for the army, Óglaigh na hÉireann. One of these, based around the Co Tyrone town of Strabane, is classified by the security forces as "a continuing and serious threat". It has already committed one murder, of a member killed in an internal dispute. The intelligence assessment is that it is involved in targeting, recruiting and training new members, as well as trying to raise funds and obtain weapons.
A second group called ONH gave a newspaper interview last month, claiming responsibility for an abortive car-bomb attack in Castlewellan, Co Down. It said it had been formed three years earlier, claiming it was made up mostly of former members of the Provisional IRA but also included ex-Real IRA and ex-Irish National Liberation Army (INLA) members. Although the attack did not work, the group boasted of its capabilities, saying: "This bomb was very much viable and so that speaks for itself as to our bomb-making expertise ... We have experts in our ranks and that will become apparent in the future."
The head of the Special Branch, Assistant Chief Constable Drew Harris, said recently that explosive devices had shown "some improvement in terms of sophistication" over the past eight or nine months. He believed, he said, that this might be due to one individual or perhaps a handful but, he added: "There is no sense of any drift across from what would have been the Provisional IRA to the dissidents. We don't see that at all."
The INLA is dangerous but relatively inactive. An originally Marxist grouping which has been in existence since the 1970s, it killed the senior Conservative MP Airey Neave at Westminster in 1979. In recent years it has not targeted the security forces, concentrating instead on activities such as "fundraising" and punishment attacks. But it maintains a capacity for violence.
Active in Northern Ireland: Dissident republican cells
Claimed responsibility for the fatal shooting of PC Stephen Carroll on Monday. Formed following a split from the IRA in 1986, it only went public and "active" in 1994, with a gun salute over the grave of the IRA commander Tom Maguire. Targets police officers and recently caught the attention of security forces for two internal "disciplinary" killings.
Claimed responsibility for Saturday night's pizza delivery attack in Antrim which killed two soldiers. Murdered 29 people and injured hundreds in the 1998 Omagh car bombing, one year after it was formed in a breakaway from the IRA's main executive. Objects to political engagement and aims to unite Ireland through militarism. Responsible for a Semtex attack last year.
ONH ( Óglaigh na hÉireann)
A term for the Irish army, this name has been adopted by various Irish paramilitary units but is currently used by at least two dissident republican cells, one of them a splinter from the Continuity IRA. A "continuing and serious threat", it has killed a member and is known to be actively recruiting.