Emissions trading (also known as cap and trade, emissions trading scheme or ETS) is a market-based approach to controlling pollution by providing economic incentives for reducing the emissions of pollutants. A central authority (usually a governmental body) allocates or sells a limited number 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.