Deep ecology is an environmental philosophy that promotes the inherent worth of all living beings regardless of their instrumental utility to human needs, and argues that modern human societies should be restructured in accordance with such ideas.

Deep ecologists argue that the natural world is a complex of relationships in which the existence of organisms is dependent on the existence of others within ecosystems. They argue that non-vital human interference with or destruction of the natural world poses a threat not only to humans, but to all organisms that make up the natural order.

Deep ecology's core principle is the belief that the living environment as a whole should be respected and regarded as having certain basic moral and legal rights to live and flourish, independent of its instrumental benefits for human use. Deep ecology is often framed in terms of the idea of a much broader sociality: it recognizes diverse communities of life on Earth that are composed not only through biotic factors but also, where applicable, through ethical relations, that is, the valuing of other beings as more than just resources. It is described as "deep" because it is regarded as looking more deeply into the reality of humanity's relationship with the natural world, arriving at philosophically more profound conclusions than those of mainstream environmentalism. The movement does not subscribe to anthropocentric environmentalism (which is concerned with conservation of the environment only for exploitation by and for human purposes), since deep ecology is grounded in a different set of philosophical assumptions. Deep ecology takes a holistic view of the world humans live in and seeks to apply to life the understanding that the separate parts of the ecosystem (including humans) function as a whole. The philosophy addresses core principles of different environmental and green movements and advocates a system of environmental ethics advocating wilderness preservation, non-coercive policies encouraging human population decline, animism and simple living.

In his original 1973 deep ecology paper, Arne Næss stated that he was inspired by ecologists who were studying the ecosystems throughout the world. Næss also made clear that he felt the real motivation to 'free nature' was spiritual and intuitive. 'Your motivation comes from your total view or your philosophical, religious opinions,' he said, 'so that you feel, when you are working in favour of free nature, you are working for something within your self, that ... demands changes. So you are motivated from what I call 'deeper premises'.

In a 2014 essay, environmentalist George Sessions identified three people active in the 1960s whom he considered foundational to the movement: author and conservationist Rachel Carson, environmentalist David Brower, and biologist Paul R. Ehrlich. Sessions considers the publication of Carson's 1962 seminal book Silent Spring as the beginning of the contemporary deep ecology movement. Næss also considered Carson the originator of the movement, stating "Eureka, I have found it" upon encountering her writings.

Another event in the 1960s which have been proposed as foundational to the movement are the images of the Earth floating in space taken by the Apollo astronauts.

Deep ecology proposes an embracing of ecological ideas and environmental ethics (that is, proposals about how humans should relate to nature). It is also a social movement based on a holistic vision of the world. Deep ecologists hold that the survival of any part is dependent upon the well-being of the whole, and criticise the narrative of human supremacy, which they say has not been a feature of most cultures throughout human evolution. Deep ecology presents an eco-centric (Earth-centred) view, rather than the anthropocentric (human-centred) view, developed in its most recent form by philosophers of the Enlightenment, such as Newton, Bacon, and Descartes. Proponents of deep ecology oppose the narrative that man is separate from nature, is in charge of nature, or is the steward of nature, or that nature exists as a resource to be freely exploited. They cite the fact that indigenous peoples under-exploited their environment and retained a sustainable society for thousands of years, as evidence that human societies are not necessarily destructive by nature. They believe that the current materialist paradigm must be replaced - as Næss pointed out, this involves more than merely getting rid of capitalism and the concept of economic growth, or 'progress', that is critically endangering the biosphere. 'We need changes in society such that reason and emotion support each other,' he said. '... not only a change in a technological and economic system, but a change that touches all the fundamental aspects of industrial societies. This is what I mean by a change of 'system'.

Deep ecologists believe that the damage to natural systems sustained since the industrial revolution now threatens social collapse and possible extinction of humans, and are striving to bring about the kind of ideological, economic and technological changes Næss mentioned. Deep ecology claims that ecosystems can absorb damage only within certain parameters, and contends that civilization endangers the biodiversity of the Earth. Deep ecologists have suggested that the human population must be substantially reduced, but advocate a gradual decrease in population rather than any apocalyptic solution In a 1982 interview, Arne Næss commented that a global population of 100 million (0.1 billion) would be desirable. However, others have argued that a population of 1 - 2 billion would be compatible with the deep ecological worldview. Deep ecology eschews traditional left wing-right wing politics, but is viewed as radical ('Deep Green') in its opposition to capitalism, and its advocacy of an ecological paradigm. Unlike conservation, deep ecology does not advocate the controlled preservation of the landbase, but rather 'non-interference' with natural diversity except for vital needs. In citing 'humans' as being responsible for excessive environmental destruction, deep ecologists actually refer to 'humans within civilization, especially industrial civilization', accepting the fact that the vast majority of humans who have ever lived did not live in environmentally destructive societies – the excessive damage to the biosphere has been sustained mostly over the past hundred years.