Ammonium nitrate (AN), an inorganic solid oxidizer used in rocket propulsion decades ago, has been enjoying a new prosperity. The main reasons favoring its return are that it produces environmentally friendly combustion products, it is very cheap, and it is a safe energetic material.The principal objectives of this work are to improve the general understanding of pure AN combustion, clarify the burning properties of a broad range of AN-based formulations, and elucidate some instability effects of pure AN, aswell as AN-based propellant burning. Experiments were performed on several categories of AN-based compositions: neat AN burning as a solidmonopropellant,with minor additives to promote decomposition chemical reactions; mixtures of AN with organic substances including trinitrotoluene; solid mixtures of AN with metallic powders; and AN water solutions with an organic fuel (also calledwater-impregnated compositions).Preliminary results indicate that, up to pressures of the order of hundreds of atmospheres, the main reactions responsible for the heat release occur mostly in the condensed phase. At higher pressures, the gas-phase  ame plays an important role in controlling the burning rate.