Energy is what both people and economies run on, so the presence of adequate energy supplies is of the utmost importance to a country or other entity. Providing energy is not as cheap as it once was, as the countries the world over have painfully found out. Whether the energy source is oil or fuelwood, it is typically the case that gathering new supplies is more difficult and require more resources. Thus, the people whose duty it is to plan for tomorrow's energy supply need to have information as to the likely nature of tomorrow's energy demand. Having too little energy may hinder economic development or even jeopardize human subsistence; Having too much means there was an unnecessary expenditure of scarce resources.