Albert Einstein's famous equation E=mc 2 shows that energy and mass are different forms of the same thing. Loosely speaking, you can convert a small amount of mass into a large amount of energy (as in a nuclear power plant , where large atoms split apart and give off energy in the process). Einstein's equation shows us we sometimes need to factor mass into the conservation of energy. In a nuclear reaction, we start off with one set of atoms (a certain amount of energy in the form of mass) and end up with a different set of atoms (a different amount of energy locked in their mass) plus energy that's released as heat. If we factor in the mass of the atoms before and after the reaction, plus the energy released in the process, we find the conservation of energy is satisfied exactly. Since mass is a form of energy, it's clear that we can't destroy mass or create it out of nothing in the same way that we can't create or destroy energy. You'll sometimes see this referred to as the conservation of mass .