An Independent on Sunday investigation suggested that almost 14,000 people a year are injured in knife attacks. Some authorities believe the number may be much higher, because many people do not report their injuries for fear of reprisals.
In gangs, like mobs, people will often do things they may never do on their own. In the UK, gangs are made up most often of boys or young men, aged 12-15 or 12-18, drawn mostly from the same or similar ethnic groups.
And gangs are by nature extremely territorial. Members will do almost anything - even commit murder - to protect an area, street or neighbourhood that they call their own.
For some young people, gangs represent excitement, risk, adventure. For others, they provide protection and belonging: there's prestige in wearing the gang's colours. For some, of course, gangs are about making money - which is why gang culture and drug dealing are so closely intertwined.
Some governments, in the UK and Europe generally, have tried to deal with the problem of youth violence with so-called zero tolerance penalties, including tougher prison sentences.
There's no doubt that, at least in London's case, more visible 'bobbies on the beat' might provide some deterrent against crimes of this kind. CCTV cameras can't take the place of police personnel. In fact, in many areas of the capital, many of the cameras don't work and those that do are relatively useless when it comes to actually preventing a crime.
Tougher law enforcement measures may also help to get some guns off the street, which would be great. Yet on their own, these measures sometimes seem to make matters worse. It's as if violent kids feel the need to hit back even harder at a system that's out to get them.
Before we can find lasting solutions to the violence problem, we need to try to understand the psychology of violence. Some people who work on the front lines with violent teenagers put them into two categories: the "initiators" and the "imitators".
Initiators learn violence in an abusive or neglectful home. Sustained abuse affects their ability to exercise control, to express empathy and see the consequences of their actions.
In the end, they can't channel their emotions in a healthy way. In fact, studies have shown that with these initiator kids, it's the act of violence itself that has a calming or soothing effect.
