Valuation of biological resources includes the process of deriving a monetary value to the things that are not sold in the market. Examples are the value given to fuelwood gathered in the forest, water filtration provided in a wetland or biological resource that may supply new medicines in the future. Valuation is essential for a "social cost- benefit" analysis of biodiversity (i.e. analyzing the costs and benefits of maintaining biodiversity for a society), and is linked to "environmental accounting" which means the modification of national accounts to take into consideration the economic role of the environment. Many countries in the world are now attempting this. It is believed that over 25 countries have experimented with environmental accounting over the past 20 years and a few European countries have already set up physical accounting systems that are routinely complied and applied to economic and environmental policy making. Biodiversity valuation can contribute to constructing environmental accounts at the national level.