This book describes the postulates (principles) of Energy Theory, which state the following. The Universe, in all of its parts ranging from the minimal to the maximal, is constantly pulsating, compressing energy in some parts and accordingly rarefying it in others. The Universe is to be considered in terms of three independent constituents (general coordinates): Energy, Space, and Time. The essence (main feature) of the Universe is the motion of Energy and Energy fields in Space-Time, nonexistent separately from each other. In the absence of motion, no fields exist; in the absence of fields, no motion is present. Energy must be divided into Material and Spiritual energies. Relative motion of material energy bunches is accessible to the human sensory system (possibly enhanced by special-purpose instruments), but relative motion of spiritual energy bunches is inaccessible to the human senses. Some of the conventional physical theories are reconsidered in the light of the proposed postulates (principles) in the Addendum. The book is intended for both physicists and the general reader interested in the subject.