The development of high explosives could be reviewed as TNT, RDX, and HMX as three historical stages, and now we are coming in the fourth stage which marked by the appearance of CL-20. However, the increment of energy denicty of the CL-20 is only about 10% greater than that of HMX.

For the needs of the improvement of the weapon systems in the next century, it is necessary to search and develop the superior high energy density materials, and it would be some quite new classes of substance.

Performance of nitrogen cluster, such as octaazacubane N₈, was calculated by quantum chemists and demonstrated its remarkable prospects, which is represented as an example of the imaginary SHEDM. And it was emphasized that new allotropic forms of nitrogen N_n would be the most promissing substance as SHEDM. Recently N₅⁺ was found in the MS, and it is already prepared as a salt by Christe.

Nitrogen polymer N_n, in which n is a large number and N atoms are connected by covalent bonds in a three dimentioal net structure just as a polymer, is should be paid the greatest attention and would become the most powerful SHEMD in the next century.