Nowadays, there is a big concern about the energy resources all over the world. Energy is a very popular word, but it is a very abstract concept. It is a well-known fact that energy cannot be created nor destroyed. It can readily be converted from one form into another. We should not be concerned about running out of energy. Energy is a property of matter and is everywhere around us. For example, one pound of air as ideal gas at standard temperature of 77oF and pressure of 1 atm. contains a tremendous amount of energy (91.51 Btu/lb). The real question here should be: How much useful work could be obtained from this energy? Surprisingly, the work potential of this huge energy reservoir, the atmosphere, is near zero. We want to bring to the table a discussion about the engineering and economic realities in the context of constantly increasing demands for energy. Several examples and sample calculations will demonstrate that we must be concerned not only with the quantity, but also with the quality of the energy.