Outlaw motorcycle gangs are well established and feared worldwide. The two best-known gangs are the Hell's Angels and the Outlaws. These gangs are separate entities to motorcycle clubs, whose members are enthusiasts of motorbikes and not linked to criminality.

Interpol describes outlaw motorcycle gangs as "one of Europe's faster-growing criminal networks" and closely monitors their activities. They are crime syndicates involved primarily in drug trafficking. Members themselves deny they are as powerful and feared as the police and media depict them, insisting that "a few bad apples" within the clubs have given them a bad name.

Motorcycle crime gangs are unique in that they maintain websites and have published by-laws by which they live their lives. Adherents pride themselves on nonconformity to society and, above all else, on loyalty to their gangs.

The Pagans, the Hells Angels, and the Bandidos have been defined as outlaw motorcycle gangs by the FBI and the Criminal Intelligence Service Canada. The authorities regard them as criminal enterprises. In the US, motorbike-gang trade in illegal goods is worth $1bn annually, according to the FBI. The gangs are some of the main distributors of illegal drugs – particularly methamphetamine – in the US.

In 1985, a three-year, 11-state FBI operation named Roughrider culminated in the largest bust in motorbike gang history. Some $2m worth of illegal drugs were confiscated as well as a massive arsenal of weapons, ranging from Uzi submachine guns to anti-tank weapons.

In 2008, the FBI embarked on a six-month undercover operation into investigating drug trafficking by the Mongols motorcycle gang. Over 100 arrests and several significant drugs seizures were made.

Canada was home to one of the most violent turf wars in motorcycle gang history. The Quebec biker war lasted from 1994 to 2002. It started when the Hells Angels in Quebec tried to take control of low-level drug dealing in the province. Several criminals resisted this and even established groups – the Alliance to Fight the Angels, and the Rock Machine – to oppose them.

The violence, which included several bombings, was sustained and bloody. More than 150 lives – including those of innocent by­standers – were lost. An 11-year-old boy was killed in one incident when he was standing beside a jeep that was blown up. A police investigation has resulted in over 100 bikers being convicted over the turf war. In 2009, over 150 members of the Hells Angels were arrested in Quebec, New Brunswick, France and the Dominican Republic, mainly in connection to crimes related to this feud.

To become a fully-fledged member of a motorcycle outlaw gang, allegiance must be sworn to the club. To earn the right to display all the club's unique patches on their jackets, prospective members must carry out acts of criminality – in some cases including rape.

Some motorcyclists describe themselves as "one-percenters" and wear patches to that effect. These signify their outlaw status from mainstream motorcycle society – they consider themselves the 1% that operates outside the law. One-percenter motorcycle clubs do not allow women to become full members. Women are submissive to the men, treated as property, and there have been widescale instances of forced prostitution and sexual abuse.

Outlaw motorcycle gangs are expanding in membership and popularity. Gang members in the US have moved to European countries and opened club chapters, recruiting members from the biking fraternity.

In recent years, gang warfare between motorcyclists has been making headlines in Australia. The Nomads, Rebels, Commancheros and Bandidos have been waging war against each other and there have been dozens of murders, drive-by shootings and firebombings. In 2007, the Australian police began a major clampdown on these gangs to try and stem the violence. Authorities have identified 35 outlaw motorcycle gangs in the country, with 3,500 members. Ten of these gangs opened 26 new chapters in Australia in just one year, a testament to the growing popularity of biker gang culture.

Closer to home, violent incidents, including murders, between biker gangs have occurred in Britain. More recently, some criminal elements of outlaw motorcycle gangs have been making inroads in Ireland. Gardaí and police in the North have been briefed by Interpol in relation to this, it is understood. The situation is being monitored closely.