This categoiy' includes both those taxes which have been explicitly introduced for environmental reasons and the much broader group of taxes, such as those on motor fuels and other energy products, which have significant environmental implications, even if the principal purpose of the tax is revenue generation. Regardless of the original motivation of the tax, it can have environmental effects through its influence on the behaviour of producers and consumers. Some other environmentally related product taxes have been introduced with the intention to provide a source of earmarked revenues for environmental purposes (e.g. to contribute revenues to an environmental clean-up fund). Generally the aim of this is to achieve behavioural changes, in the form of a shift of production or consumption away from dirty goods towards cleaner ones.