The law of conservation of energy is an empirical law of physics. A consequence of this law is that energy can neither be created or destroyed: it can only be transformed from one state to another. The only thing that can happen to energy in a closed system is that it can change form: for instance chemical energy can become kinetic energy.

Albert Einstein's theory of relativity shows that mass is a form of energy (see rest mass energy) so they can transform one into another. So the distinction between "matter" particles (such as those constituting atoms) and energy particles (say photons of light) is not wide - they can turn one into other (see annihilation and matter creation). However, this conversion does not affect the total energy of closed systems. So conservation of energy refers to the conservation of the total energy of an isolated system over time (including rest mass energy mc² associated with the rest mass of particles) and all other forms of energy (kinetic, potential, nuclear, chemical, thermal, etc) in the system together. A consequence of the law of energy conservation is that perpetual motion machines can only work perpetually if they deliver no energy to their surroundings.