The culture and civilization of a people depends primarily on the amount of energy at its disposal, and therefore continuous searches are made for new sources of energy. These searches are intimately related to questions of the transformation and conservation of energy. The fundamental laws of physics set definite limits within which these questions can be answered. It is well known that the laws of thermodynamics prevent one making perpetual motion machines of the first and second kind. But there are other restrictions imposed by the laws of physics on the practical use of energy processes. This paper considers a number of processes of energy transformation in which the energy flux density (the Poynting vector) is restricted, which precludes the possibility of using them in practice to obtain high powers. A number of energy sources are discussed as examples–thermal, chemical, biological, and finally, nuclear. The restrictions delineate the most promising directions for searching for new powerful sources of energy.