Green taxes (also called "environmental taxes" or "pollution taxes") are excise taxes on environmental pollutants or on goods whose use produces such pollutants. Economic theory suggests that taxes on polluting emissions will reduce environmental harm in the least costly manner, by encouraging changes in behavior by those firms and households that can reduce their pollution at the lowest cost. In practice, green taxes-even indirect ones, on proxies for emissions or on related goods-have rarely been imposed. Some examples can be found in Europe, but virtually none in the United States.