This chapter considered the use of methods that involve solid masses for the storage of energy. These methods fall into two general categories, those that utilize gravitation potential energy and those that utilize rotational kinetic energy. In the first case, various methods of raising masses to a higher gravitational potential for energy storage are discussed. The stored energy is then recovered by lowering the masses. Such methods are generally used for the storage of electrical energy. Energy can also be stored by rotating a flywheel and recovering the energy by decreasing its rotational rate. Such methods can be used to store electrical energy for recovery either as electrical energy or for direct use of the mechanical energy.