Eco-terrorism refers to acts of terrorism, violence or sabotage committed in support of ecological, environmental, or animal rights causes against persons or their property. Eco-terrorism is defined by the FBI as "the use or threatened use of violence of a criminal nature against people or property by an environmentally-oriented, subnational group for environmental-political reasons, or aimed at an audience beyond the target, often of a symbolic nature." The FBI has credited to eco-terrorism 200 million dollars in property damage from 2003 and 2008, and a majority of states within the USA have introduced laws aimed at eco-terrorism

Eco-terrorism is a form of radical environmentalism that arose out of the same school of thought that brought about deep ecology, ecofeminism, social ecology, and bioregionalism.  Eco-terrorism is closely related to civil disobedience and sabotage in the name of the environment, and there is a debate on where to draw the lines between the three. Not all of those who are labeled as eco-terrorists perpetuate violence against humans or animals, but instead against property. This has led to a debate that touches on whether or not to classify these actions as ‘terrorist’. In the United States the FBI’s definition includes acts of violence against property, which makes most acts of sabotage fall in the realm of domestic terrorism. Sabotage involves destroying, or threatening, to destroy property, and in this case is also known as monkeywrenching or ecotage. Many acts of sabotage involve the damage of equipment and unmanned facilities using arson.

There are a wide variety of tactics that have been used by eco-terrorists and groups associated with eco-terrorism. Examples include:

• Tree spiking is a common tactic that was first used in 1984 by members of EarthFirst. Tree spiking involves hammering a small spike into the trunk of a tree that may be logged with the intention of damaging the chainsaw or mill blades.

• Tree sitting is described as “a form of nonviolent civil disobedience in which activists reside in the upper branches of trees for long periods of time to prevent loggers from cutting them down.”

• Arson is a tactic most associated with recent activity in the Earth Liberation Front (ELF). The ELF has been attributed with arsons of sites such as housing developments, SUV dealerships, and chain stores.

• Bombing is an extreme tactic used by eco-terrorists. One of the most prominent examples of the use of explosives in the name of the environment is that of Theodore Kaczynski, better known as the Unabomber. Kaczynski’s bombing campaign lasted from 1978 to 1995—he injured 23 and killed 3 people.

Eco-terrorist organizations are generally grassroots organizations, do not have a hierarchal structure, and typically favor direct action approaches to their goals.

Dr. Stefan Leader characterizes these groups, namely ELF, with having ‘leaderless resistance’ which he describes as “a technique by which terrorist groups can carry out violent acts while reducing the risk of infiltration by law enforcement elements. The basic principal of leaderless resistance is that there is no centralized authority or chain-of-command.” Essentially this consists of independent cells which operate autonomously, sharing goals, but having no central leaders or formal organizational structure. Those who wish to join are typically encouraged to start their own cell, rather than seek out other members and jeopardize their secrecy (Leader & Probst, 2005).

Organizations that have been labeled as "eco-terrorists" in the United States include the Animal Liberation Front (ALF), the Earth Liberation Front (ELF), Greenpeace, the Sea Shepherd Conservation Society, EarthFirst!. Greenpeace generally protests through civil disobedience or sabotage and has also been implicated (and in some cases indicted) in eco-terrorism and associated unlawful use of monies as well as anti-piracy laws concerning unlawful boarding of private vessels on the high seas.

In a 2002 testimony to the US Congress, an FBI official mentioned the actions of Sea Shepherd Conservation Society in the context of eco-terrorism. The Sea Shepherd Conservation Society historically protests whaling and in 1986 caused nearly 1.8 million dollars worth of damage to equipment used by whalers in Iceland In 1992, they attacked two Japanese ships that were drift-net fishing for squid by cutting their nets and throwing bombs on board the boats. Inspired by Edward Abbey, EarthFirst! began in 1980 and has led tree spiking campaigns

The Earth Liberation Front, founded in 1992, joined with the Animal Liberation Front, which had its beginnings in England in 1979 They have been connected primarily with arson but claim that they work to harm neither human nor animal. A recent example of arson that was attributed to the ELF was in March of 2008 concerning the “torching of luxury homes in the Seattle suburb of Woodinville". A banner was left at the scene that claimed the housing development was not green, as advertised and was signed ELF. In September of 2009 the destruction of two radio towers in Seattle was also attributed to the ELF. The FBI in 2001 named the ELF as "one of the most active extremist elements in the United States", and a "terrorist threat," although they publicly disavow harm to humans or animals.

The Coalition to Save the Preserves was mentioned in FBI testimony as a group that was responsible for a series of arsons in Arizona. Using similar tactics to the ELF, they have caused more than $5 million in damages.

The thought behind eco-terrorism rises from the radical environmentalism movement, which gained popularity during the 1960s Ideas that arose from radical environmentalism are “based on the belief that capitalism, patriarchal society, and the Judeo-Christian tradition were responsible for the despoliation of nature”. Radical environmentalism is also characterized by the belief that human society is responsible for the depletion of the environment and, if current society is let unchecked, will lead to the ultimate complete degradation of the environment. Generally, nonviolence is a central tenet of radical environmentalism.

Like deep ecologists, eco-terrorists subscribe to the idea of biocentrism, which is described as “a belief that human beings are just an ordinary member of the biological community” and that all living things should have rights and deserve protection under the law. Some eco-terrorists are motivated by other aspects of deep ecology, like the goal to return the environment to its natural, pre-industrial state. Despite these generalizations, it should be noted that eco-terrorism encompasses a broad array of organizations, goals, and philosophies.