Pollution is the release of harmful environmental contaminants, or the substances so released. Generally the process needs to result from human activity to be regarded as pollution. Even relatively benign products of human activity are liable to be regarded as pollution, if they precipitate negative effects later on. The nitrogen oxides produced by industry are often referred to as pollution, for example, although the substances themselves are not harmful. In fact, it is solar energy (sunlight) that converts these compounds to smog.

Pollution can take two major forms: local pollution and global pollution. In the past, only local pollution was thought to be a problem. For example, coal burning produces smoke, which in sufficient concentrations can be a health hazard. One slogan, taught in schools, was "The solution to pollution is dilution." The theory was that sufficiently diluted pollution could cause no damage. In recent decades, awareness has been rising that some forms of pollution pose a global problem. For example, human activity (primarily nuclear testing) has significantly raised the levels of background radiation, which may lead to human health problems, all over the world. Awareness of both kinds of pollution, among other things, has led to the environmentalism movement, which seeks to limit the human impact on the environment.

Whether something is pollution depends almost entirely on context. Blooms of algae and the resultant eutrophication of lakes and coastal ocean is considered pollution when it is fueled by nutrients from industrial, agricultural, or residential runoff. Heavy metals such as lead and mercury have a role in geochemical cycles (i.e. they occur as within 'nature'). These metals may also be mined and, depending on their processing, may thus be released in large concentrations into an environment previously not playing host to them. Just as the influences of anthropogenic release of these metals to the environment may be considered as 'polluting', such pollution could also occur in some areas due to either autochtonous or historic 'natural' geochemical actiity.

Carbon dioxide emissions are sometimes referred to as pollution, on the basis that these emissions have led, or are leading, to raised levels of the gas in the atmosphere and, furthermore, to harmful changes in the Earth's climate. Such claims are strongly disputed, particularly by political conservatives in Western countries and most strongly in the United States. Due to this controversy, in many contexts carbon dioxide from such sources are labelled neutrally as "emissions." See global warming for a very extensive discussion of this topic.

Traditional forms of pollution include air pollution, water pollution, and radioactive contamination while a broader interpretation of the word has led to the ideas of ship pollution, light pollution, and noise pollution.

Serious pollution sources include chemical plants, oil refineries, nuclear waste dumps, regular garbage dumps (many toxic substances are illegally dumped there), incinerators, PVC factories, car factories, plastics factories, and corporate animal farms creating huge amounts of animal waste. Some sources of pollution, such as nuclear power plants or oil tankers, can release very severe pollution when accidents occur. Some of the more common contaminants are chlorinated hydrocarbons (CFH), heavy metals like lead (in lead paint and until recently in gasoline), cadmium (in rechargeable batteries), chromium, zinc, arsenic and benzene.

Pollution is often a serious side effect in natural disasters. For example hurricanes almost always involve sewage pollution, and petrochemical pollution from overturned boats, autos, or even damage from coastal refineries is common.

Pollutants are thought to play a part in a variety of maladies, including cancer, lupus, immune diseases, allergies, and asthma. Some illnesses are named in relation with certain pollutants: for example, Minamata disease, which is caused by mercury compounds.