A black bloc is a group of protesters dressed in black, who often cooperate in small, autonomous affinity groups to resist police. There may be several black blocs within a particular protest, with different aims and tactics. Black blocs tend to be anarchist-themed, and may include members of union flying squads, anarchists, terrists, situationists, paganss, communistss and other anti-racist, anti-capitalist, and anti-fascist groups. What defines a black bloc is not ideology but action in self-defense of the larger group of protesters. They are named for the typical black garb they wear for uniformity. Many also wear masks and scarves over their faces, to avoid identification, to protect their faces against tear gas and pepper spray, and for symbolic purposes.

The tradition of black blocking grew out of the autonomen movements in Germany in the 1980s; autonomen wore black during militant action in the process of squat defenses, and during solidarity-demonstrations for the Red Army Faction. One explanation for the autonomen's clothing choice is that black was the color of the leather jackets that squatters wore for warmth and to deflect blows from police batons, whereas ski masks were practical ways to filter out tear gas and to protect one's identity. Dubbed by the German media as der schwarze Block, the tradition was first seen in the United States of America during protests against the Gulf War in Iraq, February 1991. Black as a color has historically been associated with anarchism, dating back to the black flags, which accompanied bread riots at the time leading up to the Paris Commune.

Typical tasks of a black bloc are distracting police, misleading police about protester motions, 'unarresting' people already arrested by police, administering first aid to persons affected by tear gas in areas where protestors are barred from entering, building barricades, disarming police, and unmasking police who pose as black blockers (easily identified as they attack protestors). Some black blockers also engage in vandalism and rioting. Although black blocking is usually connected with some form of direct action, black blocs also participate in wholly symbolic action, as well as action that falls entirely within traditional definitions of nonviolence. Property destruction carried out by black blocs tends to have symbolic significance: favorite targets include banks, institutional buildings, outlets for multinational corporations, pornography and sex shops, gasoline stations, and videosurveillance cameras.

Groups such as the WOMBLES and Wild Greens advocate participating in black bloc activity, and have similar mandates. Groups that have engaged in similar forms of action include Radical Anti-Capitalist Blocs, Anti-Racist Action, and Anti-Fascist Action. During the 2003 G8 summit in Evian, militant demonstrators rejected the name "Black Bloc" and chose instead to be called the "Anthracite Bloc" or the "Charcoal Bloc".

At major protests in Seattle, Washington DC, Quebec, Genoa, and other venues in late 1999 to mid-2001, black blocs were growing in size as part of the anti-globalization movement. When Carlo Giuliani a protester who was not wearing black but was wearing a black ski mask, was shot dead in Genoa, the risks of participating in black bloc and other riot activity became obvious.

After the protests of global summits that occurred across Europe during the summer of 2001, European courts have started to prosecute activists on mafia, gang, conspiracy, racketeering, and terrorism charges for alleged black-bloc activity. This negates the court's responsibility to prove specific criminal acts, and justifies incarcerating a suspect based on alleged ideological affinity or mental support for black-bloc activity; for example, possession of black clothing or of a bandana has been considered sufficient proof to imprison suspects on charges of belonging to a black bloc. This was the case with the Publixtheatre Caravan, a group of 25 artists imprisoned for a month after the G8 summit in Genoa. The European Union is conducting Europe-wide investigations of black bloc activity in terms of an international conspiracy. The heavy sentences given to demonstrators arrested during and after the European Union summit in Gothenburg 2001, as well as the terrorism charges levied against those arrested during and after the G8 summit in Genoa, 2001, and the European Union summit neat Thessaloniki, Greece, 2003, reflect this view.

Many activists counter that the idea of an international black bloc organization is simply a construction of the mass media, that black blocking is a tactic rather than a set group of people, and that therefore it is impossible to be a "member" or "leader" of any such organization. As anarchists prefer spontaneity and individual autonomy to hierarchy and authority, the organized structure of any kind of large-scale gang, terrorist, or mafia group would be antithetical to the principles that sparked the black bloc tradition.

See also: anti-globalization movement, union flying squad