A sustainable resource is a resource that is used up at the same speed that it is renewed. Wood can be a sustainable resource if the trees are harvested at the same rate as new trees mature. Wind is a sustainable resource as the wind is not "used" up. Solar and hydroelectric power can be considered sustainable. Oil, natural gas, minerals taken from the earth do not regenerate and are non-sustainable. fisch stocks can be renewed and can be used in a sustainable fashion but are presently being used at a greater rate than they are being renewed and are considered non-sustainable.
