Hydrazinium nitroformate (HNF) has been under renewed development since the late 1980s. These research and development efforts have demonstrated that it is possible to produce HNF crystals with acceptable morphology and stability. Although the production of HNF is based on a simple acid-base reaction, the resulting product strongly depends on the synthesis and crystallization techniques used. For a propellant oxidizer, control of size (distribution) in the production phase of the crystals is important. The effect of different crystallization processes has been investigated, and the results are reported. Stability depends, among other factors, on purity and/or contamination. A promising method to determine the hydrazine and nitroform content of HNF is being discussed. Because it is demonstrated that excess hydrazine in HNF strongly affects HNF stability, such a method is important for quality control. Sonocrystallization allows improving the HNF morphology. In addition, sonocrystallization also led to improved thermal stability of the oxidizer. Cocrystallization permits the inclusion of ballistic modifiers or stabilizers in the crystals or the application of coatings during the crystallization process. This process may also increase the tap density. The slow natural decomposition of HNF during storage is demonstrated to be a first-order reaction; it permits calculating that, at room temperature, HNF may be safely stored for hundreds of years. The primary HNF decomposition products and the most likely initial decomposition/combustion mechanism have been identified, which helps in understanding the overall decomposition process. The surface temperature of burning HNF has been measured; it is strongly dependent on the pressure at which the combustion takes place. Finally, it was demonstrated that several ingredients affect the burning rate exponent of neat HNF.