Emissions trading is a market-oriented approach to controlling pollution by providing economic incentives for reducing the emissions of pollutants. The concept is also known as cap and trade (CAT) or emissions trading scheme (ETS). One prominent example is carbon emission trading for CO₂ and other greenhouse gases which is a tool for climate change mitigation. Other schemes include sulfur dioxide and other pollutants.

In an emissions trading scheme, a central authority or governmental body allocates or sells a limited number (a "cap") of permits that allow a discharge of a specific quantity of a specific pollutant over a set time period. Polluters are required to hold permits in amount equal to their emissions. Polluters that want to increase their emissions must buy permits from others willing to sell them.

Emissions trading is a type of flexible environmental regulation that allows organizations and markets to decide how best to meet policy targets. This is in contrast to command-and-control environmental regulations such as best available technology (BAT) standards and government subsidies.

Pollution is a prime example of a market externality. An externality is an effect of some activity on an entity (such as a person) that is not party to a market transaction related to that activity. Emissions trading is a market-based approach to address pollution. The overall goal of an emissions trading plan is to minimize the cost of meeting a set emissions target. In an emissions trading system, the government sets an overall limit on emissions, and defines permits (also called allowances), or limited authorizations to emit, up to the level of the overall limit. The government may sell the permits, but in many existing schemes, it gives permits to participants (regulated polluters) equal to each participant's baseline emissions. The baseline is determined by reference to the participant's historical emissions. To demonstrate compliance, a participant must hold permits at least equal to the quantity of pollution it actually emitted during the time period. If every participant complies, the total pollution emitted will be at most equal to the sum of individual limits. Because permits can be bought and sold, a participant can choose either to use its permits exactly (by reducing its own emissions); or to emit less than its permits, and perhaps sell the excess permits; or to emit more than its permits, and buy permits from other participants. In effect, the buyer pays a charge for polluting, while the seller gains a reward for having reduced emissions.

Emissions Trading results in the incorporation of economic costs into the costs of production which incentivizes corporations to consider investment returns and capital expenditure decisions with a model that includes the price of carbon and greenhouse gases (GHG).

In many schemes, organizations which do not pollute (and therefore have no obligations) may also trade permits and financial derivatives of permits. In some schemes, participants can bank allowances to use in future periods. In some schemes, a proportion of all traded permits must be retired periodically, causing a net reduction in emissions over time. Thus, environmental groups may buy and retire permits, driving up the price of the remaining permits according to the law of demand. In most schemes, permit owners can donate permits to a nonprofit entity and receive a tax deductions. Usually, the government lowers the overall limit over time, with an aim towards a national emissions reduction target.

There are active trading programs in several air pollutants. An earlier application was the US national market to reduce acid rain. The United States now has several regional markets in nitrogen oxides.

The efficiency of what later was to be called the "cap-and-trade" approach to air pollution abatement was first demonstrated in a series of micro-economic computer simulation studies between 1967 and 1970 for the National Air Pollution Control Administration (predecessor to the United States Environmental Protection Agency's Office of Air and Radiation) by Ellison Burton and William Sanjour. These studies used mathematical models of several cities and their emission sources in order to compare the cost and effectiveness of various control strategies. Each abatement strategy was compared with the "least-cost solution" produced by a computer optimization program to identify the least-costly combination of source reductions in order to achieve a given abatement goal. In each case it was found that the least-cost solution was dramatically less costly than the same amount of pollution reduction produced by any conventional abatement strategy. Burton and later Sanjour along with Edward H. Pechan continued improving and advancing these computer models at the newly created U.S. Environmental Protection Agency. The agency introduced the concept of computer modeling with least-cost abatement strategies (i.e., emissions trading) in its 1972 annual report to Congress on the cost of clean air. This led to the concept of "cap and trade" as a means of achieving the "least-cost solution" for a given level of abatement.