This paper reviews the production, synthesis, crystallography, particle morphology and spectroscopy of the insensitive high explosive nitroguanidine, (NGu, CH4N4O2), CAS-No: [556-88-7] and its isotopologues [D-2(4)]-NGu and [N-15(4)]-NGu]. When compared with standard insensitive high explosives such as 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), 1,1-diamino-2,2-dinitroethylene (FOX-7) and N-guanylurea dinitramide (FOX-12), Nitroguanidine proves to be the least sensitive. The review gives 170 references to the public domain. For Part II see ref. [1].